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]

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) – 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 p

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, 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 belo

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[16] It determines which styles apply to an element when more than one rule could apply. Based on specification, a simple selector (e.g. H1) has a specificity of 1, class selectors have a specificity of 1,0, and ID selectors 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 following rules selectors result in the indicated specificity:

Selectors	Specificity
h1 {color: white;}	0, 0, 0, 1
p em {color: green;}	0, 0, 0, 2
.grape {color: red;}	0, 0, 1, 0
p.bright {color: blue;}	0, 0, 1, 1
p.bright em.dark {color: yellow;}	0, 0, 2, 2
#id218 {colorThus the following rules selectors result in the indicated specificity: 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. 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.<!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. InheritanceInheritance 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 langua 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. Given the following style sheet: h1 { color: pink; } < h1 { color: pink; } Suppose there is an h1 element with an emphasizing element (em) inside: <h1> This is to <em>illustrate</em> inheritance </h1> If no color is assigned to the em element, the emphasized word "illustrate" inherits the color of the parent element, h1. The style sheet h1 has the color pink, hence, the em element is likewise pink. WhitespaceWhitespace between properties and selectors is ignored. This code snippet: body{overflow:hidden;backgroundbody{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; } Positioning CSS 2.1 defines three positioning schemes: Normal flow Inline items are laid out in the same There are four 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. Static The default value places the item in the normal flow Relative The item is placed in the normal flow, and then shifted or offset from that position. Subsequent flow items are laid out as if the item had not been moved. Absolute Specifies absolute positioning. The element is positioned in relation to its nearest non-static ancestor. Fixed The item is absolutely positioned in a fixed position on the screen even as the rest of the document is scrolled[20] 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. 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. left The item floats to the left of the line that it would have appeared in; other items may flow around its right side. right The item floats to the right of the line that it would have appeared in; other items may flow around its left side. clear Forces the element to appear underneath ('clear') floated elements to the left (clear:left), right (clear:right) or both sides (clear:both).Håkon Wium Lie on October 10, 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.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. 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 backwards-compatible mode of CSS interpretation ('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 backwards-compatible mode of CSS interpretation ('quirks mode') alongside an alternative, corrected 'standards mode'. Other non-Microsoft browsers also provided such 'mode'-switch behavior capability. 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 IE5/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 standard 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. CSS has various levels and profiles. Each level of CSS builds upon the last, typically adding new features and typically denoted 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 C The first CSS specification to become an official W3C Recommendation is CSS level 1, published on December 17, 1996. Håkon Wium Lie and Bert Bos are credited as the original developers.[39][40] 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 [41] 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, suppor 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)[42] and bidirectional text, and new font properties such as shadows. The W3C no longer maintains the CSS 2 recommendation.[43] CSS 2.1 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). Adoption of new functionality in CSS can be hindered by lack of support in major browsers. For example, Internet Explorer was slow to add support for many CSS 3 features, which slowed adoption of those features and damaged the browser's reputation among developers.[60] 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 Mozilla Developer Network. Additionally, the 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.[61] 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).[62] 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: Selectors are unable to ascend CSS currently offers no way to select a parent or ancestor of an element that satisfies certain criteria.[63] CSS Selectors Level 4, which is still in Working Draft status, proposes such a selector,[64] but only as part of the "complete" selector profile, not the "fast" profile used in dynamic CSS styling.[65] A more advanced selector scheme (such as XPath) would enable more sophisticated style sheets. The major reasons for the CSS Working Group previously rejecting proposals for parent selectors are related to browser performance and incremental rendering issues.[66] Cannot explicitly declare new scope independently of position Scoping rules for properties such as z-index look for the closest parent element with a position:absolute or position:relative attribute. This odd coupling has undesired effects. For example, it is impossible to avoid declaring a new scope when one is forced to adjust an element's position, preventing one from using the desired scope of a parent element. Pseudo-class dynamic behavior not controllable CSS implements pseudo-classes that allow a degree of user feedback by conditional application of alternate styles. One CSS pseudo-class, ":hover", is dynamic (equivalent of JavaScript "onmouseover") and has potential for misuse (e.g., implementing cursor-proximity popups),[67] but CSS has no ability for a client to disable it (no "disable"-like property) or limit its effects (no "nochange"-like values for each property). Cannot name rules There is no way to name a CSS rule, which would allow (for example) client-side scripts to refer to the rule even if its selector changes. Cannot include styles from a rule into another rule CSS styles often must be duplicated in several rules to achieve a desired effect, causing additional maintenance and requiring more thorough testing. Some new CSS features were proposed to solve this, but were abandoned afterwards.[68][69] Cannot target specific text without altering markup Besides the :first-letter pseudo-element, one cannot target specific ranges of text without needing to utilize place-holder elements. Former issues Additionally, several more issues were present in prior versions of the CSS standard, but have been alleviated: Vertical control limitations Though horizontal placement of elements was always generally easy to control, vertical placement was frequently unintuitive, convoluted, or outright impossible. Simple tasks, such as centering an element vertically or placing a footer no higher than bottom of the viewport required either complicated and unintuitive style rules, or simple but widely unsupported rules.[63] The Flexible Box Module improved the situation considerably and vertical control is much more straightforward and supported in all of the modern browsers.[70] Older browsers still have those issues, but most of those (mainly Internet Explorer 9 and below) are no longer supported by their vendors.[71] Absence of expressions There was no standard ability to specify property values as simple expressions (such as margin-left: 10% – 3em + 4px;). This would be useful in a variety of cases, such as calculating the size of columns subject to a constraint on the sum of all columns. Internet Explorer versions 5 to 7 support a proprietary expression() statement,[72] with similar functionality. This proprietary expression() statement is no longer supported from Internet Explorer 8 onwards, except in compatibility modes. This decision was taken for "standards compliance, browser performance, and security reasons".[72] However, a candidate recommendation with a calc() value to address this limitation has been published by the CSS WG[73] and has since been supported in all of the modern browsers.[74] Lack of column declaration Although possible in current CSS 3 (using the column-count module),[75] layouts with multiple columns can be complex to implement in CSS 2.1. With CSS 2.1, the process is often done using floating elements, which are often rendered differently by different browsers, different computer screen shapes, and different screen ratios set on standard monitors. All of the modern browsers support this CSS 3 feature in one form or another.[76] Advantages Separation of content 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, Cascade Framework, 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.[77] 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), oCSS (organic Cascade Style Sheet), SMACSS (scalable and modular architecture for CSS), and BEM (block, element, modifier).[78] References ^ "CSS developer guide". Mozilla Developer Network. Archived from the original on 2015-09-25. Retrieved 2015-09-24. ^ Flanagan, David. JavaScript - The definitive guide (6 ed.). p. 1. 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 behaviour of web pages. ^ "What is CSS?". World Wide Web Consortium. Archived from the original on 2010-11-29. Retrieved 2010-12-01. ^ "Web-based Mobile Apps of the Future Using HTML 5, CSS and JavaScript". HTMLGoodies. Archived from the original on 2014-10-20[78] References