stylesheet

A stylesheet is a file or portion of a document that specifies how content should be presented in a given medium. It separates content from presentation, enabling consistent styling across pages or documents.

In web development, stylesheets are most commonly written in CSS (Cascading Style Sheets). A stylesheet contains rules that pair selectors with property-value declarations, and the browser applies them according to the cascade, specificity, and inheritance.

Stylesheets may be linked externally, embedded in a document header, or applied inline. External stylesheets are generally preferred for maintainability and reuse.

They can target different media, such as screen or print, and may respond to conditions via media queries. Modern CSS also provides variables, layout models, and responsive design features.

Beyond CSS, the term appears in XSLT (Extensible Stylesheet Language Transformations) for transforming XML, and XSL-FO for formatting objects intended for print.

Best practices emphasize separating concerns, using semantic selectors, avoiding excessive specificity, and keeping stylesheets modular and accessible.

History notes that CSS introduced a formal stylesheet language in the 1990s, with ongoing standardization and extensions that support modern layouts and responsiveness.