CSS

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

===Style sheet===
{{Main|Style sheet (web development)}}
A style sheet consists of a list of ''rules''. Each rule or rule-set consists of one or more ''[[#Selector|selectors]]'', and a ''[[#Declaration block|declaration block]]''.

===Selector===
{{Redirect|CSS class|non-CSS use of element classes in HTML|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.

====Selector types====
Selectors may apply to the following:
*all [[HTML element|elements]] of a specific type, e.g. the second-level headers [[HTML element#Basic text|h2]]
*elements specified by [[HTML attribute|attribute]], in particular:
**''id'': an identifier unique within the document, denoted in the selector language by a [[number sign|hash]] prefix e.g. {{Code|code=#id}}
**''class'': an identifier that can annotate multiple elements in a document, denoted by a [[period (punctuation)|dot]] prefix e.g. {{Code|code=.classname}} (the phrase "CSS class", although sometimes used, is a misnomer, as element classes—specified with the [[class attribute (HTML)|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 [[Resource Description Framework|RDF]] and [[microformat]]s 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 Object Model|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====
''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 {{code|lang=css|: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 {{code|lang=css|code=a:hover}} or {{code|lang=css|code=#elementid:hover}}. 

A pseudo-class classifies document elements, such as {{code|lang=css|code=:link}} or {{code|lang=css|code=:visited}}, whereas a ''pseudo-element'' makes a selection that may consist of partial elements, such as {{code|lang=css|code=::first-line}} or {{code|lang=css|code=::first-letter}}.<ref>{{cite web |url=https://www.w3.org/TR/CSS21/selector.html#pseudo-elements |title=W3C CSS2.1 specification for pseudo-elements and pseudo-classes |publisher=World Wide Web Consortium |date=7 June 2011 |access-date=30 April 2012 |archive-url=https://web.archive.org/web/20120430011514/https://www.w3.org/TR/CSS21/selector.html#pseudo-elements |archive-date=30 April 2012 |url-status=live }}</ref> Note the distinction between the double-colon notation used for pseudo-elements and the single-colon notation used for pseudo-classes.

====Combinators====
Multiple simple selectors may be joined using combinators to specify elements by location, element type, id, class, or any combination thereof.<ref>{{cite web |url-status=live |url=https://www.w3.org/TR/CSS21/selector.html |archive-url=https://web.archive.org/web/20060423174213/https://www.w3.org/TR/CSS21/selector.html |archive-date=2006-04-23 |title=Selectors |publisher=W3C |work=Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification}}</ref> The order of the selectors is important. For example, <syntaxhighlight inline lang="css" class=nowrap>
div .myClass {color: red;}</syntaxhighlight> applies to all elements of class myClass that are inside div elements, whereas <syntaxhighlight inline lang="css" class=nowrap>.myClass div {color: red;}</syntaxhighlight> applies to all div elements that are inside elements of class myClass. This is not to be confused with concatenated identifiers such as <syntaxhighlight inline="" lang="css" class="nowrap">
div.myClass {color: red;}</syntaxhighlight> which applies to div elements of class myClass.

====Summary of selector syntax====
The following table provides a summary of selector syntax indicating usage and the version of CSS that introduced it.<ref>{{cite web |url=https://www.w3.org/TR/selectors/ |title=Selectors Level 3 |publisher=W3C |access-date=2014-05-30 |archive-url=https://web.archive.org/web/20140603165900/https://www.w3.org/TR/selectors/ |archive-date=2014-06-03 |url-status=live }}</ref>
{| class="wikitable"
|-
! Pattern !! Matches !! First defined<br/>in CSS level
|-
| {{code|2=css|1=E}} || an element of type E || 1
|-
| {{code|2=css|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
|-
| {{code|2=css|1=E:active}} || an E element during certain user actions || 1
|-
| {{code|2=css|1=E::first-line}} || the first formatted line of an E element || 1
|-
| {{code|2=css|1=E::first-letter}} || the first formatted letter of an E element || 1
|-
| {{code|2=css|1=.c}} || all elements with class="c" || 1
|-
| {{code|2=css|1=#myid}} || the element with id="myid" || 1
|-
| {{code|2=css|1=E.warning}} || an E element whose class is "warning" (the document language specifies how class is determined) || 1
|-
| {{code|2=css|1=E#myid}} || an E element with ID equal to "myid" || 1
|-
| {{code|2=css|1=.c#myid}} || the element with class="c" and ID equal to "myid" || 1
|-
| {{code|2=css|1=E F}} || an F element descendant of an E element || 1
|-
| {{code|2=css|1=*}} || any element || 2
|-
| {{code|2=css|1=E[foo]}} || an E element with a "foo" attribute || 2
|-
| {{code|2=css|1=E[foo="bar"]}} || an E element whose "foo" attribute value is exactly equal to "bar" || 2
|-
| {{code|2=css|1=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
|-
| {{code|2=css|1=E<nowiki>[foo|="en"]</nowiki>}} || an E element whose "foo" attribute has a hyphen-separated list of values beginning (from the left) with "en" || 2
|-
| {{code|2=css|1=E:first-child}} || an E element, first child of its parent || 2
|-
| {{code|2=css|1=E:lang(fr)}} || an element of type E in language "fr" (the document language specifies how language is determined) || 2
|-
| {{code|2=css|1=E::before}} || generated content before an E element's content || 2
|-
| {{code|2=css|1=E::after}} || generated content after an E element's content || 2
|-
| {{code|2=css|1=E > F}} || an F element child of an E element || 2
|-
| {{code|2=css|1=E + F}} || an F element immediately preceded by an E element || 2
|-
| {{code|2=css|1=E[foo^="bar"]}} || an E element whose "foo" attribute value begins exactly with the string "bar" || 3
|-
| {{code|2=css|1=E[foo$="bar"]}} || an E element whose "foo" attribute value ends exactly with the string "bar" || 3
|-
| {{code|2=css|1=E[foo*="bar"]}} || an E element whose "foo" attribute value contains the substring "bar" || 3
|-
| {{code|2=css|1=E:root}} || an E element, root of the document || 3
|-
| {{code|2=css|1=E:nth-child(n)}} || an E element, the n-th child of its parent || 3
|-
| {{code|2=css|1=E:nth-last-child(n)}} || an E element, the n-th child of its parent, counting from the last one || 3
|-
| {{code|2=css|1=E:nth-of-type(n)}} || an E element, the n-th sibling of its type || 3
|-
| {{code|2=css|1=E:nth-last-of-type(n)}} || an E element, the n-th sibling of its type, counting from the last one || 3
|-
| {{code|2=css|1=E:last-child}} || an E element, last child of its parent || 3
|-
| {{code|2=css|1=E:first-of-type}} || an E element, first sibling of its type || 3
|-
| {{code|2=css|1=E:last-of-type}} || an E element, last sibling of its type || 3
|-
| {{code|2=css|1=E:only-child}} || an E element, only child of its parent || 3
|-
| {{code|2=css|1=E:only-of-type}} || an E element, only sibling of its type || 3
|-
| {{code|2=css|1=E:empty}} || an E element that has no children (including text nodes) || 3
|-
| {{code|2=css|1=E:target}} || an E element being the target of the referring URI || 3
|-
| {{code|2=css|1=E:enabled}} || a user interface element E that is enabled || 3
|-
| {{code|2=css|1=E:disabled}} || a user interface element E that is disabled || 3
|-
| {{code|2=css|1=E:checked}} || a user interface element E that is checked (for instance a radio button or checkbox) || 3
|-
| {{code|2=css|1=E:not(s)}} || an E element that does not match simple selector s || 3
|-
| {{code|2=css|1=E ~ F}} || an F element preceded by an E element || 3
|}

===Declaration block===
A declaration block consists of a pair of braces (<code>{}</code>) enclosing a semicolon-separated list of ''declarations''.<ref>{{Cite web |title=CSS Syntax Module Level 3 |url=https://www.w3.org/TR/css-syntax-3/#syntax-description |url-status=live |archive-url=https://web.archive.org/web/20231001223513/https://www.w3.org/TR/css-syntax-3/ |archive-date=1 October 2023 |access-date=1 October 2023 |website=W3C}}</ref> 

====Declaration====
Each declaration itself consists of a ''property'', a colon (<code>:</code>), and a ''value''. Optional white-space may be around the declaration block, declarations, colons, and semi-colons for readability.<ref>{{cite web |url=https://www.w3.org/TR/CSS21/syndata.html#q10 |title=W3C CSS2.1 specification for rule sets, declaration blocks, and selectors |publisher=World Wide Web Consortium |date=7 June 2011 |access-date=2009-06-20 |archive-url=https://web.archive.org/web/20080328113605/https://www.w3.org/TR/CSS21/syndata.html#q10 |archive-date=28 March 2008 |url-status=live }}</ref>

====Properties====
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.<ref>{{cite web |url=https://www.w3.org/TR/CSS2/propidx.html |title=Full property table |publisher=W3C |access-date=2014-05-30 |archive-url=https://web.archive.org/web/20140530163211/https://www.w3.org/TR/CSS2/propidx.html |archive-date=2014-05-30 |url-status=live }}</ref><ref>{{Cite web|title=Index of CSS properties|url=https://www.w3.org/Style/CSS/all-properties.en.html|access-date=2020-08-09|website=W3C }}</ref>

====Values====
Values may be keywords, such as "center" or "inherit", or numerical values, such as {{code|code=200px|2=css}} (200 pixels), {{code|code=50vw|2=css}} (50 percent of the viewport width) or {{mono|80%}} (80 percent of the parent element's width).

Color values can be specified with keywords (e.g. "{{code|code=red|2=css}}"), hexadecimal values (e.g. {{code|code=#FF0000|2=css}}, also abbreviated as {{code|code=#F00|2=css}}), RGB values on a 0 to 255 scale (e.g. <code>{{code|code=rgb(255, 0, 0)|2=css}}</code>), RGBA values that specify both color and alpha transparency (e.g. {{code|code=rgba(255, 0, 0, 0.8)|2=css}}), or HSL or HSLA values (e.g. {{code|code=hsl(0 100% 50%)|2=css}}, {{code|code=hsl(0 100% 50% / 0.8)|2=css}}).<ref>{{cite web |url=https://developer.mozilla.org/en-US/docs/Web/CSS/color |title=CSS Color |publisher=MDN Web Docs |date=2024-04-05 |access-date=2024-04-05 |archive-url=https://web.archive.org/web/20240327000753/https://developer.mozilla.org/en-US/docs/Web/CSS/color |archive-date=2024-03-27 |url-status=live }}</ref>

Non-zero numeric values representing linear measures must include a length unit, which is either an alphabetic code or abbreviation, as in <code>200px</code> or <code>50vw</code>; or a percentage sign, as in <code>80%</code>. Some units – <code>cm</code> ([[centimetre]]); <code>in</code> ([[inch]]); <code>mm</code> ([[millimetre]]); <code>pc</code> ([[Pica (typography)|pica]]); and <code>pt</code> ([[Point (typography)|point]]) – are ''absolute'', which means that the rendered dimension does not depend upon the structure of the page; others – <code>em</code> ([[Em (typography)|em]]); <code>ex</code> ([[Ex (typography)|ex]]) and <code>px</code> ([[pixel]]){{clarify|reason=This "such as" is carrying too much weight. The size of a px is relative not to the font size, but to screen resolution and viewing distance. Furthermore, in CSS 3 the "px" becomes the primordial absolute unit as the length units become anchored to it.|date=March 2022}} – 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<ref>{{cite web |url=https://www.w3.org/TR/REC-CSS1-961217#length-units |title=6.1 Length units |work=Cascading Style Sheets, level 1 |date=17 December 1996 |access-date=20 June 2019 |archive-url=https://web.archive.org/web/20190614194847/https://www.w3.org/TR/REC-CSS1-961217#length-units |archive-date=14 June 2019 |url-status=live }}</ref> 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: <code>ch</code>; <code>Q</code>; <code>rem</code>; <code>vh</code>; <code>vmax</code>; <code>vmin</code>; and <code>vw</code>.<ref>{{cite web |url=https://www.w3.org/TR/2019/CR-css-values-3-20190606/#lengths |title=5. Distance Units: the &lt;length&gt; type |work=CSS Values and Units Module Level 3 |date=6 June 2019 |access-date=20 June 2019 |archive-url=https://web.archive.org/web/20190607171702/https://www.w3.org/TR/2019/CR-css-values-3-20190606/#lengths |archive-date=7 June 2019 |url-status=live }}</ref>

===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. And additionally, as more and more devices are able to access responsive web pages, different screen sizes and layouts begin to appear. Customizing a website for each device size is costly and increasingly difficult. The modular nature of CSS means that styles can be reused in different parts of a site or even across sites, promoting consistency and efficiency.

For example, headings (<code>h1</code> elements), sub-headings (<code>h2</code>), sub-sub-headings (<code>h3</code>), etc., are defined structurally using HTML. In print and on the screen, choice of [[Typeface|font]], [[Point (typography)|size]], [[color]] and [[Emphasis (typography)|emphasis]] for these elements is ''presentational''.

Before CSS, document authors who wanted to assign such [[Typography|typographic]] characteristics to, say, all <code>h2</code> 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 [[deprecation|deprecated]] the use of all presentational HTML markup.<ref>{{cite web|author=((W3C HTML Working Group)) |title=HTML 5. A vocabulary and associated APIs for HTML and XHTML|url=https://www.w3.org/TR/html/introduction.html#presentational-markup|publisher=[[World Wide Web Consortium]]|access-date=28 June 2014|archive-url=https://web.archive.org/web/20140715001359/https://www.w3.org/TR/html/introduction.html#presentational-markup|archive-date=15 July 2014|url-status=live}}</ref>

For example, under pre-CSS HTML, a heading element defined with red text would be written as:
<syntaxhighlight lang="html">
<h1><font color="red">Chapter 1.</font></h1>
</syntaxhighlight>

Using CSS, the same element can be coded using style properties instead of HTML presentational attributes:
<syntaxhighlight lang="html">
<h1 style="color: red;">Chapter 1.</h1>
</syntaxhighlight>

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:

<syntaxhighlight lang="html">
<style>
    h1 {
        color: red;
    }
</style>
</syntaxhighlight>

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

<syntaxhighlight lang="html">
<style>
    h1 {
        color: blue;
    }
</style>
</syntaxhighlight>

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

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

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

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, or Cascading Style Sheets, offers a flexible way to style web content, with styles originating from browser defaults, user preferences, or web designers. These styles can be applied inline, within an HTML document, or through external .css files for broader consistency. Not only does this simplify web development by promoting reusability and maintainability, it also improves site performance because styles can be offloaded into dedicated .css files that browsers can cache. Additionally, even if the styles cannot be loaded or are disabled, this separation maintains the accessibility and readability of the content, ensuring that the site is usable for all users, including those with disabilities. Its multi-faceted approach, including considerations for selector specificity, rule order, and media types, ensures that websites are visually coherent and adaptive across different devices and user needs, striking a balance between design intent and user accessibility.

====Multiple style sheets====

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.

====Cascading====
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 [[Style sheet (web development)#Customization|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 (browser extension)|Stylus]] have been created to facilitate the management of such user style sheets. In the case of large projects, cascading can be used to determine which style has a higher priority when developers do integrate third-party styles that have conflicting priorities, and to further resolve those conflicts. Additionally, cascading can help create themed designs, which help designers fine-tune aspects of a design without compromising the overall layout.  

=====CSS priority scheme=====
{| class="wikitable"
|+ CSS priority scheme (highest to lowest)
|-
! Priority !! CSS source type !! Description
|-
| 1 || Importance || The "{{code|lang=css|code=!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 ({{code|lang=css|code=#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'' refers to the relative weights of various rules.<ref name="Cascading">{{Cite book | edition = 3rd | publisher = O'Reilly Media, Inc. | isbn = 0-596-52733-0 | last = Meyer | first = Eric A. | title = Cascading Style Sheets: The Definitive Guide | url = https://shop.oreilly.com/product/9781565926226.do | year = 2006 | access-date = 2014-02-16 | archive-url = https://web.archive.org/web/20140215041138/https://shop.oreilly.com/product/9781565926226.do | archive-date = 2014-02-15 | url-status = live }}</ref> 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"<ref>{{cite web|url=https://www.w3.org/TR/CSS21/cascade.html#specificity|title=Assigning property values, Cascading, and Inheritance|access-date=2014-06-10|archive-url=https://web.archive.org/web/20140611010536/https://www.w3.org/TR/CSS21/cascade.html#specificity|archive-date=2014-06-11|url-status=live}}</ref> (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:
{| class="wikitable"
|-
! Selectors !! Specificity
|-
| <syntaxhighlight lang="css" inline>h1 {color: white;}</syntaxhighlight> || 0, 0, 0, 1
|-
| <syntaxhighlight lang="css" inline>p em {color: green;}</syntaxhighlight> || 0, 0, 0, 2
|-
| <syntaxhighlight lang="css" inline>.grape {color: red;}</syntaxhighlight> || 0, 0, 1, 0
|-
| <syntaxhighlight lang="css" inline>p.bright {color: blue;}</syntaxhighlight> || 0, 0, 1, 1
|-
| <syntaxhighlight lang="css" inline>p.bright em.dark {color: yellow;}</syntaxhighlight> || 0, 0, 2, 2
|-
| <syntaxhighlight lang="css" inline>#id218 {color: brown;}</syntaxhighlight> || 0, 1, 0, 0
|-
| <syntaxhighlight lang="css" inline>style=" "</syntaxhighlight> || 1, 0, 0, 0
|}

====Examples====

Consider this HTML fragment:
<syntaxhighlight lang="html">
<!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>
</syntaxhighlight>
In the above example, the declaration in the <code>style</code> attribute overrides the one in the <code><style></code> element because it has a higher specificity, and thus, the paragraph appears green:

<div class="html-box" style="min-width: 160pt; height: 80pt; background-color: white; border: 2pt solid #303030; padding: 5px; user-select: none;>
     <p id="xyz" style="color: green;">To demonstrate specificity</p>

</div>

===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.<ref name="Cascading"/> 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 [[Class-based programming#Inheritance|inheritance in class-based programming languages]], where it is possible to define class B as "like class A, but with modifications".<ref>{{Cite web|title = Can a CSS class inherit one or more other classes?|url = https://stackoverflow.com/questions/1065435/can-a-css-class-inherit-one-or-more-other-classes|website = StackOverflow|access-date = 2017-09-10|archive-url = https://web.archive.org/web/20171014054727/https://stackoverflow.com/questions/1065435/can-a-css-class-inherit-one-or-more-other-classes|archive-date = 2017-10-14|url-status = live}}</ref> 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:
<syntaxhighlight lang="css">
p {
   color: pink;
}
</syntaxhighlight>
Suppose there is a p element with an emphasizing element (&lt;em&gt;) inside:
<syntaxhighlight lang="html">
<p>
   This is to <em>illustrate</em> inheritance
</p>
</syntaxhighlight>
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:

<div class="html-box" style="min-width: 160pt; height: 80pt; background-color: white; border: 2pt solid #303030; padding: 5px; user-select: none;>
  <p style="color: pink;">This is to <em>illustrate</em> inheritance</p>

</div>

===Whitespace===
The whitespace between properties and selectors is ignored. This code snippet:
<syntaxhighlight lang="CSS">
body{overflow:hidden;background:#000000;background-image:url(images/bg.gif);background-repeat:no-repeat;background-position:left top;}
</syntaxhighlight>
is functionally equivalent to this one:
<syntaxhighlight lang="CSS">
body {
   overflow: hidden;
   background-color: #000000;
   background-image: url(images/bg.gif);
   background-repeat: no-repeat;
   background-position: left top;
}
</syntaxhighlight>

====Indentation====
{{Main|Indentation style}}
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:<ref name="Mozilla Developers">{{cite news |url=https://developer.mozilla.org/en-US/docs/Web/CSS/Shorthand_properties |title=Shorthand properties |work=Tutorial |publisher=Mozilla Developers |date=2017-12-07 |access-date=2018-01-30 |archive-url=https://web.archive.org/web/20180130204516/https://developer.mozilla.org/en-US/docs/Web/CSS/Shorthand_properties |archive-date=2018-01-30 |url-status=dead }}</ref>

<syntaxhighlight lang="CSS">
body {
   overflow: hidden;
   background: #000 url(images/bg.gif) no-repeat left top;
}
</syntaxhighlight>Sometimes, multiple property values are indented onto their own line:<syntaxhighlight lang="css">
@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');
}
</syntaxhighlight>

===Positioning===
CSS&nbsp;2.1 defines three positioning schemes:
; Normal flow: ''Inline'' items are laid out in the same way as the letters in words in the text, one after the other across the available space until there is no more room, then starting a new line below. ''Block'' items stack vertically, like paragraphs and like the items in a bulleted list. Normal flow also includes the relative positioning of block or inline items and run-in boxes.
; Floats: A floated item is taken out of the normal flow and shifted to the left or right as far as possible in the space available. Other content then flows alongside the floated item.
; Absolute positioning: An absolutely positioned item has no place in, and no effect on, the normal flow of other items. It occupies its assigned position in its container independently of other items.<ref name="W3C-positioning">{{cite web|last=Bos|first=Bert|title=9.3 Positioning schemes|url=https://www.w3.org/TR/CSS2/visuren.html#positioning-scheme|work=Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification|publisher=W3C|access-date=16 February 2011|date=7 December 2010|display-authors=etal|archive-url=https://web.archive.org/web/20110218054848/https://www.w3.org/TR/CSS2/visuren.html#positioning-scheme|archive-date=18 February 2011|url-status=live}}</ref>

====Position property====
There are five possible values of the <syntaxhighlight lang="CSS" inline>position</syntaxhighlight> property. If an item is positioned in any way other than <syntaxhighlight lang="CSS" inline>static</syntaxhighlight>, then the further properties <syntaxhighlight lang="CSS" inline>top</syntaxhighlight>, <syntaxhighlight lang="CSS" inline>bottom</syntaxhighlight>, <syntaxhighlight lang="CSS" inline>left</syntaxhighlight>, and <syntaxhighlight lang="CSS" inline>right</syntaxhighlight> are used to specify offsets and positions.The element having position static is not affected by the  <syntaxhighlight lang="css" inline="">top</syntaxhighlight>, <syntaxhighlight lang="css" inline="">bottom</syntaxhighlight> , <syntaxhighlight lang="css" inline="">left</syntaxhighlight> or  <syntaxhighlight lang="css" inline="">right</syntaxhighlight> properties.

=====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<ref name="W3C-positioning"/>

====Float and clear====
The {{Code|float|css}} 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 {{Code|clear|css}} 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 ({{code|lang=css|code=clear:left}}), right ({{code|lang=css|code=clear:right}}) or both sides ({{code|lang=css|code=clear:both}}).<ref name="W3C-positioning"/><ref>{{cite book|last=Holzschlag|first=Molly E|author-link=Molly Holzschlag|title=Spring into HTML and CSS|year=2005|publisher=Pearson Education, Inc|isbn=0-13-185586-7}}</ref>