<bold> tag indicating how such text should be displayed. This separation of formatting and content makes it possible to present the same markup page in different styles for different rendering methods, such as on-screen, in print, by voice (when read out by a speech-based browser or screen reader) and on Braille-based, tactile devices. It can also be used to display the web page differently depending on the screen size or device on which it is being viewed. While the author of a web page typically links to a CSS file within the markup file, readers can specify a different style sheet, such as a CSS file stored on their own computer, to override the one the author has specified. If the author or the reader did not link the document to a style sheet, the default style of the browser will be applied. Another advantage of CSS is that aesthetic changes to the graphic design of a document (or hundreds of documents) can be changed quickly and easily, by editing a few lines in one file, rather than by a laborious (and thus expensive) process of crawling over every document line by line, changing markup. The CSS specification describes a priority scheme to determine which style rules apply if more than one rule matches against a particular element. In this so-called cascade, priorities or weights are calculated and assigned to rules, so that the results are 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 (March 1998). The W3C operates a free CSS validation service for CSS documents. 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.
In CSS, selectors are used to declare which part of the markup a style applies to by matching tags and attributes in the markup itself. Selectors may apply to:
- all elements of a specific type, e.g. the second level headers h2
- elements specified by attribute, in particular:
- id: an identifier unique to the document
- class: an identifier that groups multiple elements in a document
- 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 and underscores. Any number of instances of any number of elements may have the same class. Conventionally, IDs only apply to one instance of one 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.
Selectors may be combined in many ways to achieve great specificity and flexibility. 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 in elements of class myClass.
The following table provides a summary of selector syntax indicating usage and the version of CSS in which it was introduced.
| Pattern | Matches | First Defined in CSS Level |
|---|---|---|
| E | An element of type E | 1 |
| E:link | an E element being the source anchor of a hyperlink of which the target is not yet visited (:link) or already 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 |
| 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 |
| 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 sub-string "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 which is enabled | 3 |
| E:disabled | a user interface element E which is disabled | 3 |
| E:checked | a user interface element E which 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.
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.
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.
Values may be keywords, such as "center" or "inherit", or numerical values, such as 200px (200 pixels) or 80%. 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 opacity (e.g. rgba(255, 0, 0, 0.8)), or HSL or HSLA values (e.g. hsl(000, 100%, 50%), hsla(000, 100%, 50%, 80%)).Use.
Before CSS, nearly all of the 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 allows authors to move much of that information to another file, the style sheet, resulting in considerably simpler HTML.
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 the speed and emphasis with which text is read out by aural text readers. The W3C has now deprecated the use of all presentational HTML markup.
For Example, Under Pre-CSS HTML, A Header 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>
An "external" CSS file, as described below, can be associated with an HTML document using the following syntax:
<link href="path/to/file.css" rel="stylesheet">
An internal CSS code can be typed in the head section of the code. The coding is started with the style tag. For example,
<style>
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 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. This 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).
| High Priority | CSS Source Type | Description |
|---|---|---|
| 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 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.
Specificity describes the relative weights of various rules. It determines which styles are applied 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" (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;} | 1 |
| P EM {color: green;} | 2 |
| .grape {color: red;} | 1,0 |
| P.bright {color: blue;} | 1,1 |
| P.bright EM.dark {color: yellow;} | 2,2 |
| #id218 {color: brown;} | 1,0,0 |
| style=" " | 1,0,0,0 |
Example Of Specificity.
Consider this HTML fragment:
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <style> #xyz { color: red } </style> </head> <body> <p id="xyz" style="color: blue"> To demonstrate specificity </p> </body> </html>
In the above example, the declaration in the "style" attribute will override the one in the style element since it has a higher 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. Inheritance relies on the document tree, which is the hierarchy of (X)HTML 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 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 prevents certain properties from being declared over and over again in a style sheet, allowing the software developers to write less CSS. It enhances faster-loading of web pages by users and enables the clients to save money on bandwidth and development costs.
Example Of Inheritance.
Given the following style sheet:
h1 { color: pink; }
Suppose there is an h1 element with an emphasizing element (em) inside:
<h1> This to <em>illustrate</em> inheritance </h1>
If no color has been assigned to the em element, the emphasized word "illustrate" will inherit the color of the parent element, h1. The style sheet h1 has the color pink, hence, the em element will likewise be in pink.
Whitespace.
Whitespace between properties and selectors is ignored. This code snippet:
body{overflow:hidden;background:#000000;}
is functionally equivalent to this one:
body { overflow: hidden; background: #000000; }
One common way to format CSS for readability is to indent each property and give it its own line.