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

{{Main|Style sheet (web development)}}

A style sheet consists of a list of rules. Each rule or rule-set consists of one or more selectors, and a declaration block.

{{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.

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. {{Code|code=#id}}
• class: an identifier that can annotate multiple elements in a document, denoted by a dot prefix e.g. {{Code|code=.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 element. 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 {{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}}.{{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 }} Note the distinction between the double-colon notation used for pseudo-elements and the single-colon notation used for pseudo-classes.

Multiple simple selectors may be joined using combinators to specify elements by location, element type, id, class, or any combination thereof.{{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}} 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.{{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 }}

A declaration block consists of a pair of braces ({}) enclosing a semicolon-separated list of declarations.{{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}}

Each declaration itself consists of a property, a colon (:), and a value. Optional white-space may be around the declaration block, declarations, colons, and semi-colons for readability.{{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 }}

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.{{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 }}{{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 }}

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=rgb(255, 0, 0)|2=css}}), 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}}).{{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 }}

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){{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{{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 }} 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.{{cite web |url=https://www.w3.org/TR/2019/CR-css-values-3-20190606/#lengths |title=5. Distance Units: the <length> 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 }}

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 (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 reading speed and emphasis for aural text readers. The W3C has now deprecated the use of all presentational HTML markup.{{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}}

For example, under pre-CSS HTML, a heading element defined with red text would be written as:

Chapter 1.

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

Chapter 1.

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:

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:

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 below, and loaded using syntax similar to:

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.

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 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.

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 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 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.

Specificity refers to the relative weights of various rules.{{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 }} 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"{{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}} (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:

Consider this HTML fragment:

To demonstrate specificity

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