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What is css in e

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Cascading Style Sheets (CSS)

What is css in e

The official logo of the latest version, CSS 3

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Filename extension

.css

Internet media type

text/css

Uniform Type Identifier (UTI)public.css
Developed byWorld Wide Web Consortium (W3C)
Initial release17 December 1996; 25 years ago
Latest release

CSS 2.1 : Level 2 Revision 1
12 April 2016; 6 years ago

Type of formatStyle sheet language
Container forStyle rules for HTML elements (tags)
Contained byHTML Documents
Open format?Yes
Websitewww.w3.org/TR/CSS/#css

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 presentation and content, 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[edit]

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[edit]

"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 elements. 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]

PatternMatchesFirst defined
in CSS level
E an element of type E 1
E:link an E element that is the source anchor of a hyperlink whose target is either not yet visited (:link) or already visited (:visited) 1
E:active an E element during certain user actions 1
E::first-line the first formatted line of an E element 1
E::first-letter the first formatted letter of an E element 1
.c all elements with class="c" 1
#myid the element with id="myid" 1
E.warning an E element whose class is "warning" (the document language specifies how class is determined) 1
E#myid an E element with ID equal to "myid" 1
.c#myid the element with class="c" and ID equal to "myid" 1
E F an F element descendant of an E element 1
* any element 2
E[foo] an E element with a "foo" attribute 2
E[foo="bar"] an E element whose "foo" attribute value is exactly equal to "bar" 2
E[foo~="bar"] an E element whose "foo" attribute value is a list of whitespace-separated values, one of which is exactly equal to "bar" 2
E[foo|="en"] an E element whose "foo" attribute has a hyphen-separated list of values beginning (from the left) with "en" 2
E:first-child an E element, first child of its parent 2
E:lang(fr) an element of type E in language "fr" (the document language specifies how language is determined) 2
E::before generated content before an E element's content 2
E::after generated content after an E element's content 2
E > F an F element child of an E element 2
E + F an F element immediately preceded by an E element 2
E[foo^="bar"] an E element whose "foo" attribute value begins exactly with the string "bar" 3
E[foo$="bar"] an E element whose "foo" attribute value ends exactly with the string "bar" 3
E[foo*="bar"] an E element whose "foo" attribute value contains the substring "bar" 3
E:root an E element, root of the document 3
E:nth-child(n) an E element, the n-th child of its parent 3
E:nth-last-child(n) an E element, the n-th child of its parent, counting from the last one 3
E:nth-of-type(n) an E element, the n-th sibling of its type 3
E:nth-last-of-type(n) an E element, the n-th sibling of its type, counting from the last one 3
E:last-child an E element, last child of its parent 3
E:first-of-type an E element, first sibling of its type 3
E:last-of-type an E element, last sibling of its type 3
E:only-child an E element, only child of its parent 3
E:only-of-type an E element, only sibling of its type 3
E:empty an E element that has no children (including text nodes) 3
E:target an E element being the target of the referring URI 3
E:enabled a user interface element E that is enabled 3
E:disabled a user interface element E that is disabled 3
E:checked a user interface element E that is checked (for instance a radio-button or checkbox) 3
E:not(s) an E element that does not match simple selector s 3
E ~ F an F element preceded by an E element 3

Declaration block[edit]

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[edit]

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[edit]

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 (h2 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: 

<h2><font color="red">Chapter 1.font>h2>

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

<h2 style="color: red;">Chapter 1.h2>

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>
    h2 {
        color: red;
    }
style>

All h2 elements in the document will then automatically become red without requiring any explicit code. If the author later wanted to make h2 elements blue instead, this could be done by changing the style element to: 

<style>
    h2 {
        color: blue;
    }
style>

rather than by laboriously going through the document and changing the color for each individual h2 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[edit]

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 that 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 web site, 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.

CSS priority scheme (highest to lowest)
PriorityCSS source typeDescription
1 Importance The "!important" annotation overwrites the previous priority types
2 Inline A style applied to an HTML element via HTML "style" attribute
3 Media Type A property definition applies to all media types, unless a media specific CSS is defined
4 User defined Most browsers have the accessibility feature: a user defined CSS
5 Selector specificity A specific contextual selector (#heading p) overwrites generic definition
6 Rule order Last rule declaration has a higher priority
7 Parent inheritance If a property is not specified, it is inherited from a parent element
8 CSS property definition in HTML document CSS rule or CSS inline style overwrites a default browser value
9 Browser default The lowest priority: browser default value is determined by W3C initial value specifications

Specificity[edit]

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 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:

SelectorsSpecificity
h2 {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 {color: brown;} 0, 1, 0, 0
style=" " 1, 0, 0, 0

Examples[edit]

Consider this HTML fragment: 


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

In the above example, the declaration in the style attribute overrides the one in the