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The module presents the concepts used in the CSS programming language.


Cascading Style Sheets (abbreviated CSS) are used to describe how (X)HTML or XML text is to be presented. This module is currently a commented link list of important entry points for reading about CSS.

This module gives a simple example how a CSS style sheet works.

Content of this module

Css rules

A CSS file consists of statements which are either at-rules or rule sets (often just called rules). A rule set (rule) consists of a selector and a declaration block which contains one or more declarations (more see CSS 2.1 specification ). A declaration contains a property and a value (e.g. "color" and "green"), see example below. These rules describe how the formatting should take place.

Below are some sample rules. In the first part one or several selectors are given and in the second part there are one or more attribute-value pairs enclosed by curly brackets.

/* Sample rules */ h1, h2 {color: green}#box {width: 70%} .topic {color: red}

    Selector types

  • h1 and h2 are tag selectors
  • #box is a selector for an identification
  • .topic is a selector for a class

A series of tests for selectors.

Internal and external style sheets

A style sheet may reside in a separate file ( external style sheet ) or within an (X)HTML file ( internal style sheet ).

External style sheet

<head><link rel="stylesheet" type="text/css" media="screen, projection, handheld, print" href="css/general.css" /><link rel="stylesheet" type="text/css" media="print" href="css/ourPrintStyleSheet.css" /></head>

In the case of the code above taken from a HTML file we load a general style sheet for different kinds of output media called general.css and a specific one for printing ( ourPrintStyleSheet.css ). The style sheet for printing adds and overrides some rules to put the content onto paper.

Internal stylesheet (html)

<style type="text/css">h1 { color: green;}</style>

Internal stylesheet (xhtml)

<style type="text/css"><![CDATA[ h1 {color: green; }]]></style>

@import rule

The @import rule ( ref. ) allows you to import rules from another style sheet into your current one. For an example see ´web fonts´ below.


CSS 2.1 specification (implemented by most browsers); an index

All W3C CSS standards and drafts

The link above includes the documents commonly called CSS 3 which consists of different modules. They are implemented to various degrees in the current browsers.


w3.org: Starting with HTML and CSS (basic introduction)

Complete course with interactive exercises w3schools

Searching for the key words css cheat sheet provides helpful links if you have learned CSS in the past. This brings up for example this list which leads to 30 examples .

Layout with css

For doing layout with CSS the 'position' attribute and the box model are used. In the past this was difficult because the box model implemented by the Internet Explorer (IE) differed from the web standard and from the box model implemented by other browsers. In the meantime the Internet Explorer supports the standard W3C box model so using CSS in modern browsers is made easier as there needs not to be a 'switch' anymore for IE and the other browsers. CSS 3 introduces the box-sizing property which allows to set how the box size is calculated ( (External Link) ).

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Source:  OpenStax, Test collection revision. OpenStax CNX. May 02, 2012 Download for free at https://legacy.cnx.org/content/col11421/1.1
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