Stylesheets

A stylesheet is a set of rules that describes the presentation of a document. In web contexts, stylesheets separate content from presentation, allowing the same HTML to be presented in different ways. The most widely used stylesheet language is CSS, standardized for styling HTML and XML documents. Stylesheets are applied by the browser to determine color, layout, typography, and other visual aspects.

Formats and usage: The primary approach is CSS. Styles can be embedded in a document, included via internal style blocks, linked as external files, or delivered inline with style attributes. CSS supports media types such as screen and print, enabling different rules for different output contexts. Other stylesheet systems exist for XML and print, such as XSL-FO.

Structure and behavior: A stylesheet contains selectors that target elements and declarations that assign values to properties. The cascade merges rules from multiple sources, taking into account specificity, source order, and importance. Inheritance passes certain properties down the tree, allowing consistent styling across descendants.

Impact and tooling: Stylesheets improve maintainability and consistency, enable theming and responsive design, and support accessibility by providing user-style overrides. Developers may use preprocessors (such as Sass or Less) to extend CSS syntax, and validators to check conformance.