What is CSS HTML called?

All web standards technologies (HTML, CSS, JavaScript, etc.) are defined in giant documents called specifications (or "specs"), which are published by standards organizations (such as the W3C, WHATWG, ECMA, or Khronos) and define precisely how those technologies are supposed to behave.

CSS is no different — it is developed by a group within the W3C called the CSS Working Group. This group is made of representatives of browser vendors and other companies who have an interest in CSS. There are also other people, known as invited experts, who act as independent voices; they are not linked to a member organization.

New CSS features are developed or specified by the CSS Working Group — sometimes because a particular browser is interested in having some capability, other times because web designers and developers are asking for a feature, and sometimes because the Working Group itself has identified a requirement. CSS is constantly developing, with new features becoming available. However, a key thing about CSS is that everyone works very hard to never change things in a way that would break old websites. A website built in 2000, using the limited CSS available then, should still be usable in a browser today!

As a newcomer to CSS, it is likely that you will find the CSS specs overwhelming — they are intended for engineers to use to implement support for the features in user agents, not for web developers to read to understand CSS. Many experienced developers would much rather refer to MDN documentation or other tutorials. Nevertheless, it is worth knowing that these specs exist and understanding the relationship between the CSS you are using, the browser support (see below), and the specs.

Style sheet language

"Pseudo-element" redirects here. For pseudoelement symbols in chemistry, see Skeletal formula § Pseudoelement symbols.

For other uses, see CSS (disambiguation).

For the use of CSS on Wikipedia, see Help:Cascading Style Sheets.

The official logo of the latest version, CSS 3

.css

text/css

CSS 2.1 : Level 2 Revision 1
12 April 2016; 6 years ago (2016-04-12)

Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML).[1] CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.[2]

CSS is designed to enable the separation of content and presentation, including layout, colors, and fonts.[3] This separation can improve content accessibility; provide more flexibility and control in the specification of presentation characteristics; enable multiple web pages to share formatting by specifying the relevant CSS in a separate .css file, which reduces complexity and repetition in the structural content; and enable the .css file to be cached to improve the page load speed between the pages that share the file and its formatting.

Separation of formatting and content also makes it feasible to present the same markup page in different styles for different rendering methods, such as on-screen, in print, by voice (via speech-based browser or screen reader), and on Braille-based tactile devices. CSS also has rules for alternate formatting if the content is accessed on a mobile device.[4]

The name cascading comes from the specified priority scheme to determine which style rule applies if more than one rule matches a particular element. This cascading priority scheme is predictable.

The CSS specifications are maintained by the World Wide Web Consortium (W3C). Internet media type (MIME type) text/css is registered for use with CSS by RFC 2318 (March 1998). The W3C operates a free CSS validation service for CSS documents.[5]

In addition to HTML, other markup languages support the use of CSS including XHTML, plain XML, SVG, and XUL.

Syntax

CSS has a simple syntax and uses a number of English keywords to specify the names of various style properties.

A style sheet consists of a list of rules. Each rule or rule-set consists of one or more selectors, and a declaration block.

Selector

"CSS class" redirects here. For non-CSS use of element classes in HTML, see class attribute (HTML).

In CSS, selectors declare which part of the markup a style applies to by matching tags and attributes in the markup itself.

Selectors may apply to the following:

• all elements of a specific type, e.g. the second-level headers h2
• elements specified by attribute, in particular:
  • id: an identifier unique within the document, denoted in the selector language by a hash prefix e.g. #id
  • class: an identifier that can annotate multiple elements in a document, denoted by a dot prefix e.g. .classname (the phrase "CSS class", although sometimes used, is a misnomer, as element classes—specified with the HTML class attribute—is a markup feature that is distinct from browsers' CSS subsystem and the related W3C/WHATWG standards work on document styles; see RDF and microformats for the origins of the "class" system of the Web content model)
• elements depending on how they are placed relative to others in the document tree.

Classes and IDs are case-sensitive, start with letters, and can include alphanumeric characters, hyphens, and underscores. A class may apply to any number of instances of any element. An ID may only be applied to a single element.

Pseudo-classes are used in CSS selectors to permit formatting based on information that is not contained in the document tree. One example of a widely used pseudo-class is :hover, which identifies content only when the user "points to" the visible element, usually by holding the mouse cursor over it. It is appended to a selector as in a:hover or #elementid:hover. A pseudo-class classifies document elements, such as :link or :visited, whereas a pseudo-element makes a selection that may consist of partial elements, such as ::first-line or ::first-letter.[6] Note the double-colon notation for pseudo-elements versus single-colon notation for pseudo-class.

Selectors may be combined in many ways to achieve great specificity and flexibility.[7] Multiple selectors may be joined in a spaced list to specify elements by location, element type, id, class, or any combination thereof. The order of the selectors is important. For example, div .myClass {color: red;} applies to all elements of class myClass that are inside div elements, whereas .myClass div {color: red;} applies to all div elements that are inside elements of class myClass. This is not to be confused with concatenated identifiers such as div.myClass {color: red;} which applies to div elements of class myClass.

The following table provides a summary of selector syntax indicating usage and the version of CSS that introduced it.[8]

Declaration block

A declaration block consists of a list of declarations in braces. Each declaration itself consists of a property, a colon (:), and a value. If there are multiple declarations in a block, a semi-colon (;) must be inserted to separate each declaration. An optional semi-colon after the last (or single) declaration may be used.[9]

Properties are specified in the CSS standard. Each property has a set of possible values. Some properties can affect any type of element, and others apply only to particular groups of elements.[10][11]

Values may be keywords, such as "center" or "inherit", or numerical values, such as 200px (200 pixels), 50vw (50 percent of the viewport width) or 80% (80 percent of the parent element's width). Color values can be specified with keywords (e.g. "red"), hexadecimal values (e.g. #FF0000, also abbreviated as #F00), RGB values on a 0 to 255 scale (e.g. rgb(255, 0, 0)), RGBA values that specify both color and alpha transparency (e.g. rgba(255, 0, 0, 0.8)), or HSL or HSLA values (e.g. hsl(000, 100%, 50%), hsla(000, 100%, 50%, 80%)).[12]

Length units

Non-zero numeric values representing linear measures must include a length unit, which is either an alphabetic code or abbreviation, as in 200px or 50vw; or a percentage sign, as in 80%. Some units – cm (centimetre); in (inch); mm (millimetre); pc (pica); and pt (point) – are absolute, which means that the rendered dimension does not depend upon the structure of the page; others – em (em); ex (ex) and px (pixel)[clarification needed] – are relative, which means that factors such as the font size of a parent element can affect the rendered measurement. These eight units were a feature of CSS 1[13] and retained in all subsequent revisions. The proposed CSS Values and Units Module Level 3 will, if adopted as a W3C Recommendation, provide seven further length units: ch; Q; rem; vh; vmax; vmin; and vw.[14]

Use

Before CSS, nearly all presentational attributes of HTML documents were contained within the HTML markup. All font colors, background styles, element alignments, borders, and sizes had to be explicitly described, often repeatedly, within the HTML. CSS lets authors move much of that information to another file, the style sheet, resulting in considerably simpler HTML.

For example, headings (h1 elements), sub-headings (h2), sub-sub-headings (h3), etc., are defined structurally using HTML. In print and on the screen, choice of font, size, color and emphasis for these elements is presentational.

Before CSS, document authors who wanted to assign such typographic characteristics to, say, all h2 headings had to repeat HTML presentational markup for each occurrence of that heading type. This made documents more complex, larger, and more error-prone and difficult to maintain. CSS allows the separation of presentation from structure. CSS can define color, font, text alignment, size, borders, spacing, layout and many other typographic characteristics, and can do so independently for on-screen and printed views. CSS also defines non-visual styles, such as reading speed and emphasis for aural text readers. The W3C has now deprecated the use of all presentational HTML markup.[15]

For example, under pre-CSS HTML, a heading element defined with red text would be written as:

<h1><font color="red">Chapter 1.</font></h1>

Using CSS, the same element can be coded using style properties instead of HTML presentational attributes:

<h1 style="color: red;">Chapter 1.</h1>

The advantages of this may not be immediately clear but the power of CSS becomes more apparent when the style properties are placed in an internal style element or, even better, an external CSS file. For example, suppose the document contains the style element:

<style> h1 { color: red; } </style>

All h1 elements in the document will then automatically become red without requiring any explicit code. If the author later wanted to make h1 elements blue instead, this could be done by changing the style element to:

<style> h1 { color: blue; } </style>

rather than by laboriously going through the document and changing the color for each individual h1 element.

The styles can also be placed in an external CSS file, as described below, and loaded using syntax similar to:

<link href="path/to/file.css" rel="stylesheet" type="text/css">

This further decouples the styling from the HTML document and makes it possible to restyle multiple documents by simply editing a shared external CSS file.

Sources

CSS information can be provided from various sources. These sources can be the web browser, the user, and the author. The information from the author can be further classified into inline, media type, importance, selector specificity, rule order, inheritance, and property definition. CSS style information can be in a separate document, or it can be embedded into an HTML document. Multiple style sheets can be imported. Different styles can be applied depending on the output device being used; for example, the screen version can be quite different from the printed version, so authors can tailor the presentation appropriately for each medium.

The style sheet with the highest priority controls the content display. Declarations not set in the highest priority source are passed on to a source of lower priority, such as the user agent style. The process is called cascading.

One of the goals of CSS is to allow users greater control over presentation. Someone who finds red italic headings difficult to read may apply a different style sheet. Depending on the browser and the website, a user may choose from various style sheets provided by the designers, or may remove all added styles, and view the site using the browser's default styling, or may override just the red italic heading style without altering other attributes. Browser extensions like Stylish and Stylus have been created to facilitate the management of such user style sheets.

Specificity

Specificity refers to the relative weights of various rules.[16] It determines which styles apply to an element when more than one rule could apply. Based on the specification, a simple selector (e.g. H1) has a specificity of 1, class selectors have a specificity of 1,0, and ID selectors have a specificity of 1,0,0. Because the specificity values do not carry over as in the decimal system, commas are used to separate the "digits"[17] (a CSS rule having 11 elements and 11 classes would have a specificity of 11,11, not 121).

Thus the selectors of the following rule result in the indicated specificity:

Examples

Consider this HTML fragment:

<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <style> #xyz { color: blue; } </style> </head> <body> <p id="xyz" style="color: green;">To demonstrate specificity</p> </body> </html>

In the above example, the declaration in the style attribute overrides the one in the <style> element because it has a higher specificity, and thus, the paragraph appears green:

To demonstrate specificity

Inheritance

Inheritance is a key feature in CSS; it relies on the ancestor-descendant relationship to operate. Inheritance is the mechanism by which properties are applied not only to a specified element but also to its descendants.[16] Inheritance relies on the document tree, which is the hierarchy of XHTML elements in a page based on nesting. Descendant elements may inherit CSS property values from any ancestor element enclosing them. In general, descendant elements inherit text-related properties, but their box-related properties are not inherited. Properties that can be inherited are color, font, letter spacing, line-height, list-style, text-align, text-indent, text-transform, visibility, white-space, and word-spacing. Properties that cannot be inherited are background, border, display, float and clear, height, and width, margin, min- and max-height and -width, outline, overflow, padding, position, text-decoration, vertical-align, and z-index.

Inheritance can be used to avoid declaring certain properties over and over again in a style sheet, allowing for shorter CSS.

Inheritance in CSS is not the same as inheritance in class-based programming languages, where it is possible to define class B as "like class A, but with modifications".[18] With CSS, it is possible to style an element with "class A, but with modifications". However, it is not possible to define a CSS class B like that, which could then be used to style multiple elements without having to repeat the modifications.

Example

Given the following style sheet:

p { color: pink; }

Suppose there is a p element with an emphasizing element (<em>) inside:

<p> This is to <em>illustrate</em> inheritance </p>

If no color is assigned to the em element, the emphasized word "illustrate" inherits the color of the parent element, p. The style sheet p has the color pink, hence, the em element is likewise pink:

This is to illustrate inheritance

Whitespace

The whitespace between properties and selectors is ignored. This code snippet:

body{overflow:hidden;background:#000000;background-image:url(images/bg.gif);background-repeat:no-repeat;background-position:left top;}

is functionally equivalent to this one:

body { overflow: hidden; background-color: #000000; background-image: url(images/bg.gif); background-repeat: no-repeat; background-position: left top; }

One common way to format CSS for readability is to indent each property and give it its own line. In addition to formatting CSS for readability, shorthand properties can be used to write out the code faster, which also gets processed more quickly when being rendered:[19]

body { overflow: hidden; background: #000 url(images/bg.gif) no-repeat left top; }

Sometimes, multiple property values are indented onto their own line:

@font-face { font-family: 'Comic Sans' font-size: 20px src: url('first.example.com'), url('second.example.com'), url('third.example.com'), url('fourth.example.com'), }

Positioning

CSS 2.1 defines three positioning schemes:

Position property

There are five possible values of the position property. If an item is positioned in any way other than static, then the further properties top, bottom, left, and right are used to specify offsets and positions.The element having position static is not affected by the top, bottom , left or right properties.

Float and clear

The float property may have one of three values. Absolutely positioned or fixed items cannot be floated. Other elements normally flow around floated items, unless they are prevented from doing so by their clear property.

History

CSS was first proposed by Håkon Wium Lie on 10 October 1994.[22] At the time, Lie was working with Tim Berners-Lee at CERN.[23] Several other style sheet languages for the web were proposed around the same time, and discussions on public mailing lists and inside World Wide Web Consortium resulted in the first W3C CSS Recommendation (CSS1)[24] being released in 1996. In particular, a proposal by Bert Bos was influential; he became co-author of CSS1, and is regarded as co-creator of CSS.[25]

Style sheets have existed in one form or another since the beginnings of Standard Generalized Markup Language (SGML) in the 1980s, and CSS was developed to provide style sheets for the web.[26] One requirement for a web style sheet language was for style sheets to come from different sources on the web. Therefore, existing style sheet languages like DSSSL and FOSI were not suitable. CSS, on the other hand, let a document's style be influenced by multiple style sheets by way of "cascading" styles.[26]

As HTML grew, it came to encompass a wider variety of stylistic capabilities to meet the demands of web developers. This evolution gave the designer more control over site appearance, at the cost of more complex HTML. Variations in web browser implementations, such as ViolaWWW and WorldWideWeb,[27] made consistent site appearance difficult, and users had less control over how web content was displayed. The browser/editor developed by Tim Berners-Lee had style sheets that were hard-coded into the program. The style sheets could therefore not be linked to documents on the web.[23] Robert Cailliau, also of CERN, wanted to separate the structure from the presentation so that different style sheets could describe different presentation for printing, screen-based presentations, and editors.[27]

Improving web presentation capabilities was a topic of interest to many in the web community and nine different style sheet languages were proposed on the www-style mailing list.[26] Of these nine proposals, two were especially influential on what became CSS: Cascading HTML Style Sheets[22] and Stream-based Style Sheet Proposal (SSP).[25][28] Two browsers served as testbeds for the initial proposals; Lie worked with Yves Lafon to implement CSS in Dave Raggett's Arena browser.[29][30][31] Bert Bos implemented his own SSP proposal in the Argo browser.[25] Thereafter, Lie and Bos worked together to develop the CSS standard (the 'H' was removed from the name because these style sheets could also be applied to other markup languages besides HTML).[23]

Lie's proposal was presented at the "Mosaic and the Web" conference (later called WWW2) in Chicago, Illinois in 1994, and again with Bert Bos in 1995.[23] Around this time the W3C was already being established and took an interest in the development of CSS. It organized a workshop toward that end chaired by Steven Pemberton. This resulted in W3C adding work on CSS to the deliverables of the HTML editorial review board (ERB). Lie and Bos were the primary technical staff on this aspect of the project, with additional members, including Thomas Reardon of Microsoft, participating as well. In August 1996, Netscape Communication Corporation presented an alternative style sheet language called JavaScript Style Sheets (JSSS).[23] The spec was never finished, and is deprecated.[32] By the end of 1996, CSS was ready to become official, and the CSS level 1 Recommendation was published in December.

Development of HTML, CSS, and the DOM had all been taking place in one group, the HTML Editorial Review Board (ERB). Early in 1997, the ERB was split into three working groups: HTML Working group, chaired by Dan Connolly of W3C; DOM Working group, chaired by Lauren Wood of SoftQuad; and CSS Working group, chaired by Chris Lilley of W3C.

The CSS Working Group began tackling issues that had not been addressed with CSS level 1, resulting in the creation of CSS level 2 on November 4, 1997. It was published as a W3C Recommendation on May 12, 1998. CSS level 3, which was started in 1998, is still under development as of 2014.

In 2005, the CSS Working Groups decided to enforce the requirements for standards more strictly. This meant that already published standards like CSS 2.1, CSS 3 Selectors, and CSS 3 Text were pulled back from Candidate Recommendation to Working Draft level.

Difficulty with adoption

This section needs to be updated. Please help update this article to reflect recent events or newly available information. (January 2019)

The CSS 1 specification was completed in 1996. Microsoft's Internet Explorer 3[23] was released in that year, featuring some limited support for CSS. IE 4 and Netscape 4.x added more support, but it was typically incomplete and had many bugs that prevented CSS from being usefully adopted. It was more than three years before any web browser achieved near-full implementation of the specification. Internet Explorer 5.0 for the Macintosh, shipped in March 2000, was the first browser to have full (better than 99 percent) CSS 1 support,[33] surpassing Opera, which had been the leader since its introduction of CSS support fifteen months earlier. Other browsers followed soon afterward, and many of them additionally implemented parts of CSS 2.[citation needed]

However, even when later "version 5" web browsers began to offer a fairly full implementation of CSS, they were still incorrect in certain areas and were fraught with inconsistencies, bugs and other quirks. Microsoft Internet Explorer 5. x for Windows, as opposed to the very different IE for Macintosh, had a flawed implementation of the CSS box model, as compared with the CSS standards. Such inconsistencies and variation in feature support made it difficult for designers to achieve a consistent appearance across browsers and platforms without the use of workarounds termed CSS hacks and filters. The IE Windows box model bugs were so serious that, when Internet Explorer 6 was released, Microsoft introduced a backward-compatible mode of CSS interpretation ("quirks mode") alongside an alternative, corrected "standards mode". Other non-Microsoft browsers also provided mode-switch capabilities. It, therefore, became necessary for authors of HTML files to ensure they contained special distinctive "standards-compliant CSS intended" marker to show that the authors intended CSS to be interpreted correctly, in compliance with standards, as opposed to being intended for the now long-obsolete IE5/Windows browser. Without this marker, web browsers that have the "quirks mode"-switching capability will size objects in web pages as IE 5 on Windows would, rather than following CSS standards.[citation needed]

Problems with patchy adoption of CSS, along with errata in the original specification, led the W3C to revise the CSS 2 standards into CSS 2.1, which moved nearer to a working snapshot of current CSS support in HTML browsers. Some CSS 2 properties that no browser successfully implemented were dropped, and in a few cases, defined behaviors were changed to bring the standard into line with the predominant existing implementations. CSS 2.1 became a Candidate Recommendation on February 25, 2004, but CSS 2.1 was pulled back to Working Draft status on June 13, 2005,[34] and only returned to Candidate Recommendation status on July 19, 2007.[35]

In addition to these problems, the .css extension was used by a software product used to convert PowerPoint files into Compact Slide Show files,[36] so some web servers served all .css[37] as MIME type application/x-pointplus[38] rather than text/css.

Vendor prefixes

Individual browser vendors occasionally introduced new parameters ahead of standardization and universalization. To prevent interfering with future implementations, vendors prepended unique names to the parameters, such as -moz- for Mozilla Firefox, -webkit- named after the browsing engine of Apple Safari, -o- for Opera Browser and -ms- for Microsoft Internet Explorer and early versions of Microsoft Edge that use EdgeHTML.

Occasionally, the parameters with vendor prefixes such as -moz-radial-gradient and -webkit-linear-gradient have slightly different syntax as compared to their non-vendor-prefix counterparts.[39]

Prefixed properties are rendered obsolete by the time of standardization. Programs are available to automatically add prefixes for older browsers and to point out standardized versions of prefixed parameters. Since prefixes are limited to a small subset of browsers, removing the prefix allows other browsers to see the functionality. An exception is certain obsolete -webkit- prefixed properties, which are so common and persistent on the web that other families of browsers have decided to support them for compatibility.[40]

Variations

CSS has various levels and profiles. Each level of CSS builds upon the last, typically adding new features and typically denoted[citation needed] as CSS 1, CSS 2, CSS 3, and CSS 4. Profiles are typically a subset of one or more levels of CSS built for a particular device or user interface. Currently, there are profiles for mobile devices, printers, and television sets. Profiles should not be confused with media types, which were added in CSS 2.

CSS 1

The first CSS specification to become an official W3C Recommendation is CSS level 1, published on 17 December 1996. Håkon Wium Lie and Bert Bos are credited as the original developers.[41][42] Among its capabilities are support for

• Font properties such as typeface and emphasis
• Color of text, backgrounds, and other elements
• Text attributes such as spacing between words, letters, and lines of text
• Alignment of text, images, tables and other elements
• Margin, border, padding, and positioning for most elements
• Unique identification and generic classification of groups of attributes

The W3C no longer maintains the CSS 1 Recommendation.[43]

CSS 2

CSS level 2 specification was developed by the W3C and published as a recommendation in May 1998. A superset of CSS 1, CSS 2 includes a number of new capabilities like absolute, relative, and fixed positioning of elements and z-index, the concept of media types, support for aural style sheets (which were later replaced by the CSS 3 speech modules)[44] and bidirectional text, and new font properties such as shadows.

The W3C no longer maintains the CSS 2 recommendation.[45]

CSS 2.1

CSS level 2 revision 1, often referred to as "CSS 2.1", fixes errors in CSS 2, removes poorly supported or not fully interoperable features and adds already implemented browser extensions to the specification. To comply with the W3C Process for standardizing technical specifications, CSS 2.1 went back and forth between Working Draft status and Candidate Recommendation status for many years. CSS 2.1 first became a Candidate Recommendation on 25 February 2004, but it was reverted to a Working Draft on 13 June 2005 for further review. It returned to Candidate Recommendation on 19 July 2007 and then updated twice in 2009. However, because changes and clarifications were made, it again went back to Last Call Working Draft on 7 December 2010.

CSS 2.1 went to Proposed Recommendation on 12 April 2011.[46] After being reviewed by the W3C Advisory Committee, it was finally published as a W3C Recommendation on 7 June 2011.[47]

CSS 2.1 was planned as the first and final revision of level 2—but low-priority work on CSS 2.2 began in 2015.

CSS 3

"CSS3" redirects here. For other uses, see CSS3 (disambiguation).

Unlike CSS 2, which is a large single specification defining various features, CSS 3 is divided into several separate documents called "modules". Each module adds new capabilities or extends features defined in CSS 2, preserving backward compatibility. Work on CSS level 3 started around the time of publication of the original CSS 2 recommendation. The earliest CSS 3 drafts were published in June 1999.[48]

Due to the modularization, different modules have different stability and statuses.[49]

Some modules have Candidate Recommendation (CR) status and are considered moderately stable. At CR stage, implementations are advised to drop vendor prefixes.[50]

This article needs to be updated. Please help update this article to reflect recent events or newly available information. (January 2021)

CSS 4

"CSS4" redirects here. For other uses, see CSS4 (disambiguation).

There is no single, integrated CSS4 specification,[52] because the specification has been split into many separate modules which level independently.

Modules that build on things from CSS Level 2 started at Level 3. Some of them have already reached Level 4 or are already approaching Level 5. Other modules that define entirely new functionality, such as Flexbox,[53] have been designated as Level 1 and some of them are approaching Level 2.

The CSS Working Group sometimes publishes "Snapshots", a collection of whole modules and parts of other drafts that are considered stable enough to be implemented by browser developers. So far, five such "best current practices" documents have been published as Notes, in 2007,[54] 2010,[55] 2015,[56] 2017,[57] and 2018.[58]

Since these specification snapshots are primarily intended for developers, there has been a growing demand for a similar versioned reference document targeted at authors, which would present the state of interoperable implementations as meanwhile documented by sites like Can I Use...[59] and the MDN Web Docs.[60] A W3C Community Group has been established in early 2020 in order to discuss and define such a resource.[61] The actual kind of versioning is also up to debate, which means that the document once produced might not be called "CSS4".

Browser support

Further information: Comparison of web browser engines (CSS support)

Each web browser uses a layout engine to render web pages, and support for CSS functionality is not consistent between them. Because browsers do not parse CSS perfectly, multiple coding techniques have been developed to target specific browsers with workarounds (commonly known as CSS hacks or CSS filters). The adoption of new functionality in CSS can be hindered by a lack of support in major browsers. For example, Internet Explorer was slow to add support for many CSS 3 features, which slowed the adoption of those features and damaged the browser's reputation among developers. Additionally, a proprietary syntax for the non-vendor-prefixed filter property was used in some versions.[62] In order to ensure a consistent experience for their users, web developers often test their sites across multiple operating systems, browsers, and browser versions, increasing development time and complexity. Tools such as BrowserStack have been built to reduce the complexity of maintaining these environments.

In addition to these testing tools, many sites maintain lists of browser support for specific CSS properties, including CanIUse and the MDN Web Docs. Additionally, CSS 3 defines feature queries, which provide an @supports directive that will allow developers to target browsers with support for certain functionality directly within their CSS.[63] CSS that is not supported by older browsers can also sometimes be patched in using JavaScript polyfills, which are pieces of JavaScript code designed to make browsers behave consistently. These workarounds—and the need to support fallback functionality—can add complexity to development projects, and consequently, companies frequently define a list of browser versions that they will and will not support.

As websites adopt newer code standards that are incompatible with older browsers, these browsers can be cut off from accessing many of the resources on the web (sometimes intentionally).[64] Many of the most popular sites on the internet are not just visually degraded on older browsers due to poor CSS support but do not work at all, in large part due to the evolution of JavaScript and other web technologies.

Limitations

Some noted limitations of the current capabilities of CSS include:

Advantages

Main article: Separation of content and presentation

Main article: Style sheet (web development)

Main article: Progressive enhancement

Standardization

Frameworks

Main article: CSS framework

CSS frameworks are pre-prepared libraries that are meant to allow for easier, more standards-compliant styling of web pages using the Cascading Style Sheets language. CSS frameworks include Blueprint, Bootstrap, Foundation and Materialize. Like programming and scripting language libraries, CSS frameworks are usually incorporated as external .css sheets referenced in the HTML <head>. They provide a number of ready-made options for designing and laying out the web page. Although many of these frameworks have been published, some authors use them mostly for rapid prototyping, or for learning from, and prefer to 'handcraft' CSS that is appropriate to each published site without the design, maintenance and download overhead of having many unused features in the site's styling.[68]

Design methodologies

As the size of CSS resources used in a project increases, a development team often needs to decide on a common design methodology to keep them organized. The goals are ease of development, ease of collaboration during development, and performance of the deployed stylesheets in the browser. Popular methodologies include OOCSS (object-oriented CSS), ACSS (atomic CSS), CSS (organic Cascade Style Sheet), SMACSS (scalable and modular architecture for CSS), and BEM (block, element, modifier).[69]

See also

• Flash of unstyled content

References

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2. ^ Flanagan, David (18 April 2011). JavaScript: the definitive guide. Beijing; Farnham: O'Reilly. p. 1. ISBN 978-1-4493-9385-4. OCLC 686709345. JavaScript is part of the triad of technologies that all Web developers must learn: HTML to specify the content of web pages, CSS to specify the presentation of web pages, and JavaScript to specify the behavior of web pages.
3. ^ "What is CSS?". World Wide Web Consortium. Archived from the original on 2010-11-29. Retrieved 2010-12-01.
4. ^ "Web-based Mobile Apps of the Future Using HTML 5, CSS and JavaScript". HTMLGoodies. 23 July 2010. Archived from the original on 2014-10-20. Retrieved 2014-10-16.
5. ^ "W3C CSS validation service". Archived from the original on 2011-02-14. Retrieved 2012-06-30.
6. ^ "W3C CSS2.1 specification for pseudo-elements and pseudo-classes". World Wide Web Consortium. 7 June 2011. Archived from the original on 30 April 2012. Retrieved 30 April 2012.
7. ^ see the complete definition of selectors at the W3C Web site Archived 2006-04-23 at the Wayback Machine.
8. ^ "Selectors Level 3". W3.org. Archived from the original on 2014-06-03. Retrieved 2014-05-30.
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Further reading

• Meyer, Eric A. (2017). Cascading Style Sheets: The Definitive Guide, Fourth Edition. O'Reilly Media, Inc. ISBN 9781449393199.
• Grant, Keith J. (2018). CSS in Depth. Manning Publications Co. ISBN 9781617293450.
• MDN CSS reference
• MDN Getting Started with CSS

External links

CSS at Wikipedia's sister projects

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  Definitions from Wiktionary
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  Media from Commons
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  Textbooks from Wikibooks
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  Resources from Wikiversity
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  Data from Wikidata
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• Official website
  

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Page 2

Version 2 of the Hypertext Transfer Protocol used by the World Wide Web

HTTP/2 (originally named HTTP/2.0) is a major revision of the HTTP network protocol used by the World Wide Web. It was derived from the earlier experimental SPDY protocol, originally developed by Google.[1][2] HTTP/2 was developed by the HTTP Working Group (also called httpbis, where "bis" means "twice") of the Internet Engineering Task Force (IETF).[3][4][5] HTTP/2 is the first new version of HTTP since HTTP/1.1, which was standardized in RFC 2068 in 1997. The Working Group presented HTTP/2 to the Internet Engineering Steering Group (IESG) for consideration as a Proposed Standard in December 2014,[6][7] and IESG approved it to publish as Proposed Standard on February 17, 2015 (and was updated in February 2020 in regard to TLS 1.3).[8][9] The HTTP/2 specification was published as RFC 7540 on May 14, 2015.[10]

The standardization effort was supported by Chrome, Opera, Firefox,[11] Internet Explorer 11, Safari, Amazon Silk, and Edge browsers.[12] Most major browsers had added HTTP/2 support by the end of 2015.[13] About 97% of web browsers used have the capability.[14] As of December 2022[update], 41% (after topping out at just over 50%) of the top 10 million websites supported HTTP/2.[15]

Its successor is HTTP/3, a major revision that builds on the concepts established by HTTP/2.[2][16][17][18]

Goals

The working group charter mentions several goals and issues of concern:[4]

• Create a negotiation mechanism that allows clients and servers to elect to use HTTP/1.1, 2.0, or potentially other non-HTTP protocols.
• Maintain high-level compatibility with HTTP/1.1 (for example with methods, status codes, URIs, and most header fields).
• Decrease latency to improve page load speed in web browsers by considering:
  • data compression of HTTP headers
  • HTTP/2 Server Push
  • prioritization of requests
  • multiplexing multiple requests over a single TCP connection (fixing the head-of-line blocking problem in HTTP 1.x)
• Support common existing use cases of HTTP, such as desktop web browsers, mobile web browsers, web APIs, web servers at various scales, proxy servers, reverse proxy servers, firewalls, and content delivery networks.

Differences from HTTP/1.1

The proposed changes do not require any changes to how existing web applications work, but new applications can take advantage of new features for increased speed.[19] HTTP/2 leaves all of HTTP/1.1's high-level semantics, such as methods, status codes, header fields, and URIs, the same. What is new is how the data is framed and transported between the client and the server.[19]

Websites that are efficient minimize the number of requests required to render an entire page by minifying (reducing the amount of code and packing smaller pieces of code into bundles, without reducing its ability to function) resources such as images and scripts. However, minification is not necessarily convenient nor efficient and may still require separate HTTP connections to get the page and the minified resources. HTTP/2 allows the server to "push" content, that is, to respond with data for more queries than the client requested. This allows the server to supply data it knows a web browser will need to render a web page, without waiting for the browser to examine the first response, and without the overhead of an additional request cycle.[20]

Additional performance improvements in the first draft of HTTP/2 (which was a copy of SPDY) come from multiplexing of requests and responses to avoid some of the head-of-line blocking problem in HTTP 1 (even when HTTP pipelining is used), header compression, and prioritization of requests.[21] However, as HTTP/2 runs on top of a single TCP connection there is still potential for head-of-line blocking to occur if TCP packets are lost or delayed in transmission.[22] HTTP/2 no longer supports HTTP/1.1's chunked transfer encoding mechanism, as it provides its own, more efficient, mechanisms for data streaming.[23]

History

Genesis in and later differences from SPDY

SPDY (pronounced like "speedy") was a previous HTTP-replacement protocol developed by a research project spearheaded by Google.[24] Primarily focused on reducing latency, SPDY uses the same TCP pipe but different protocols to accomplish this reduction. The basic changes made to HTTP/1.1 to create SPDY included: "true request pipelining without FIFO restrictions, message framing mechanism to simplify client and server development, mandatory compression (including headers), priority scheduling, and even bi-directional communication".[25]

The HTTP Working Group considered Google's SPDY protocol, Microsoft's HTTP Speed+Mobility proposal (SPDY based),[24] and Network-Friendly HTTP Upgrade.[26] In July 2012, Facebook provided feedback on each of the proposals and recommended HTTP/2 be based on SPDY.[27] The initial draft of HTTP/2 was published in November 2012 and was based on a straight copy of SPDY.[28]

The biggest difference between HTTP/1.1 and SPDY was that each user action in SPDY is given a "stream ID", meaning there is a single TCP channel connecting the user to the server. SPDY split requests into either control or data, using a "simple to parse binary protocol with two types of frames".[25][29] SPDY showed evident improvement over HTTP, with a new page load speedup ranging from 11% to 47%.[30]

The development of HTTP/2 used SPDY as a jumping-off point. Among the many detailed differences between the protocols, the most notable is that HTTP/2 uses a fixed Huffman code-based header compression algorithm, instead of SPDY's dynamic stream-based compression. This helps to reduce the potential for compression oracle attacks on the protocol, such as the CRIME attack.[29]

On February 9, 2015, Google announced plans to remove support for SPDY in Chrome in favor of support for HTTP/2.[31] That took effect, starting with Chrome 51.[32][33]

Development milestones

Encryption

HTTP/2 is defined both for HTTP URIs (i.e. without TLS encryption, a configuration which is abbreviated in h2c) and for HTTPS URIs (over TLS using ALPN extension[50] where TLS 1.2 or newer is required, a configuration which is abbreviated in h2).

Although the standard itself does not require usage of encryption,[51] all major client implementations (Firefox,[52] Chrome, Safari, Opera, IE, Edge) have stated that they will only support HTTP/2 over TLS, which makes encryption de facto mandatory.[53]

Criticisms

Development process

The FreeBSD and Varnish developer Poul-Henning Kamp asserts that the standard was prepared on an unrealistically short schedule, ruling out any basis for the new HTTP/2 other than the SPDY protocol and resulting in other missed opportunities for improvement. Kamp criticizes the protocol itself for being inconsistent and having needless, overwhelming complexity. He also states that the protocol violates the protocol layering principle, for example by duplicating flow control that belongs in the transport layer (TCP). He also suggested that the new protocol should have removed HTTP Cookies, introducing a breaking change.[54]

Encryption

Initially, some members[who?] of the Working Group tried to introduce an encryption requirement in the protocol. This faced criticism.

Critics stated that encryption has non-negligible computing costs and that many HTTP applications actually have no need for encryption and their providers have no desire to spend additional resources on it. Encryption proponents have stated that this encryption overhead is negligible in practice.[55] Poul-Henning Kamp has criticized the IETF for hastily standardizing Google's SPDY prototype as HTTP/2 due to political considerations.[54][56][57] The criticism of the agenda of mandatory encryption within the existing certificate framework is not new, nor is it unique to members of the open-source community – a Cisco employee stated in 2013 that the present certificate model is not compatible with small devices like routers, because the present model requires not only annual enrollment and remission of non-trivial fees for each certificate, but must be continually repeated on an annual basis.[58] In the end the Working Group did not reach consensus over the mandatory encryption,[51] although most client implementations require it, which makes encryption a de facto requirement.

The HTTP/2 protocol also faced criticism for not supporting opportunistic encryption, a measure against passive monitoring similar to the STARTTLS mechanism that has long been available in other Internet protocols like SMTP. Critics have stated that the HTTP/2 proposal goes in violation of IETF's own RFC 7258 "Pervasive Monitoring Is an Attack", which also has a status of Best Current Practice 188.[59] RFC7258/BCP188 mandates that passive monitoring be considered as an attack, and protocols designed by IETF should take steps to protect against passive monitoring (for example, through the use of opportunistic encryption). A number of specifications for opportunistic encryption of HTTP/2 have been provided,[60][61][62] of which draft-nottingham-http2-encryption was adopted as an official work item of the working group, leading to the publication of RFC 8164 in May 2017.

TCP head-of-line blocking

Although the design of HTTP/2 effectively addresses the HTTP-transaction-level head-of-line blocking problem by allowing multiple concurrent HTTP transactions, all those transactions are multiplexed over a single TCP connection, meaning that any packet-level head-of-line blocking of the TCP stream simultaneously blocks all transactions being accessed via that connection. This head-of-line blocking in HTTP/2 is now widely regarded as a design flaw, and much of the effort behind QUIC and HTTP/3 has been devoted to reduce head-of-line blocking issues.[63][64]

Server-side support

Main article: Comparison of web server software

Server software

• Apache 2.4.12 supports HTTP/2 via the module mod_h2,[65] although appropriate patches must be applied to the source code of the server in order for it to support that module. As of Apache 2.4.17 all patches are included in the main Apache source tree, although the module itself was renamed mod_http2.[66] Old versions of SPDY were supported via the module mod_spdy,[67] however the development of the mod_spdy module has stopped.[68]
• Apache Tomcat supports HTTP/2 with version 8.5 and newer with a configuration change.[69]
• Apache Traffic Server supports HTTP/2.[70]
• Caddy supports HTTP/2.[71]
• Charles Proxy supports HTTP/2 since version Charles 4.[72]
• Citrix NetScaler 11.x supports HTTP/2.[73]
• Sucuri Supports HTTP/2.[74]
• F5 BIG-IP Local Traffic Manager 11.6 supports HTTP/2.[75]
• Barracuda Networks WAF (Web Application Firewall) supports HTTP/2.[76]
• h2o was built from the ground up for HTTP/2 support.[77]
• HAProxy 1.8 supports HTTP/2.[78]
• Jetty 9.3 supports HTTP/2.[79]
• lighttpd 1.4.56 supports HTTP/2.[80]
• LiteSpeed Web Server 5.0 supports HTTP/2.[81]
• Microsoft IIS supports HTTP/2 in Windows 10,[82] Windows Server 2016, and Windows Server 2019.
• Netty 4.1 supports HTTP/2.[83]
• nginx 1.9.5 supports HTTP/2,[84] released on September 22, 2015, using module ngx_http_v2_module and HTTP/2 Server Push since version 1.13.9 on February 20, 2018.[85]
• Node.js Stable support since 8.13.0.[86] (5.0 supports HTTP/2 with a module[87] and Node 8.4 introduced experimental built-in support for HTTP/2.[88])
• Kestrel web server for ASP.NET Core supports HTTP/2 since .NET Core 2.2.0-preview 1.[89]
• OpenLiteSpeed 1.3.11 and 1.4.8 supports HTTP/2.[90]
• Proxygen supports HTTP/2.
• Pulse Secure Virtual Traffic Manager 10.2 supports HTTP/2.[91]
• Radware Alteon NG supports HTTP/2.[92]
• ShimmerCat supports HTTP/2.[93]
• Vert.x 3.3 supports HTTP/2.
• Warp (Haskell web server, used by default in Yesod) supports HTTP/2.
• Wildfly 9 supports HTTP/2.
• Envoy proxy supports HTTP/2.

Content delivery networks

• Akamai was the first major CDN to support HTTP/2 and HTTP/2 Server Push.
• Microsoft Azure supports HTTP/2.
• PageCDN supports HTTP/2 out of the box and provides user-interface to setup HTTP/2 Server Push in CDN dashboard.[94]
• CDN77 supports HTTP/2 using nginx (August 20, 2015).
• Cloudflare supports HTTP/2 using nginx with SPDY as a fallback for browsers without support, whilst maintaining all security and performance services.[95] Cloudflare was the first major CDN to support HTTP/2 Server Push.[96]
• AWS CloudFront supports HTTP/2[97] since September 7, 2016.
• Fastly supports HTTP/2 including Server Push.[98]
• Imperva Incapsula CDN supports HTTP/2.[99] The implementation includes support for WAF and DDoS mitigation features as well.
• KeyCDN supports HTTP/2 using nginx (October 6, 2015). HTTP/2 Test is a test page to verify if your server supports HTTP/2.
• Voxility supports HTTP/2 using nginx since July, 2016. The implementation comes in support for Cloud DDoS mitigation services.[100]
• StackPath supports HTTP/2.

Implementations

• Other implementations are collected on the GitHub HTTP/2 wiki.

See also

• gRPC
• HTTP pipelining
• HTTP request and response messages
• HTTP/3
• QUIC
• SPDY
• WebSocket
• Web Server
• Web Browser
• Comparison of web browsers § Protocol support

References

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2. ^ a b Cimpanu, Catalin. "HTTP-over-QUIC to be renamed HTTP/3 | ZDNet". ZDNet. Retrieved November 19, 2018.
3. ^ Thomson, M. (ed.), Belshe M. and R. Peon. "Hypertext Transfer Protocol version 2: draft-ietf-httpbis-http2-16". ietf.org. HTTPbis Working Group. Retrieved February 11, 2015.
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10. ^ "RFC 7540 - Hypertext Transfer Protocol Version 2 (HTTP/2)". IETF. May 2015. Retrieved May 14, 2015.
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External links

• Official website
  
• HTTP/2 on GitHub
• RFC 7540 – Hypertext Transfer Protocol version 2 (HTTP/2)
• RFC 7541 – HPACK: Header Compression for HTTP/2
• HTTP/2 explained (Daniel Stenberg)
• SPDY Protocol (draft-mbelshe-httpbis-spdy-00)
• HTTP Speed+Mobility (draft-Montenegro-httpbis-speed-mobility-01)
• Proposal for a Network-Friendly HTTP Upgrade (draft-tarreau-httpbis-network-friendly-00)

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