CSSs

CSS stands for Cascading Style Sheets, a style-sheet language used to describe the presentation of documents written in HTML, XML, and other markup languages. It enables control over layout, typography, colors, spacing, and visual effects, separating content from presentation. Stylesheets, or CSS modules, can be external files, embedded in the document head, or applied inline.

CSS works through a system of selectors that target elements and declarations that set property values. A rule-set consists of a selector and a block of declarations, such as color, font-size, margin, and display. The cascade combines rules from different sources and determines the final value based on origin, importance, specificity, and source order. Inheritance allows some properties to be passed from parent to child elements.

Styles can be authored in external stylesheets linked to documents, embedded in a style element, or placed directly on elements as inline styles. Responsive design is achieved with media queries that apply different rules depending on viewport size or other media features. Modern CSS also includes custom properties (variables), functions, and layout modules such as Flexbox and Grid to create complex and flexible layouts.

CSS has evolved through several standards starting with CSS1 and CSS2, with CSS3 introducing modular features and long-term evolution into CSS4 concepts. Browser engines implement CSS with varying levels of support, leading to the use of vendor prefixes and progressive enhancement. The ecosystem includes preprocessors (for example, Sass and Less) and frameworks or methodologies (such as Bootstrap or utility-based approaches) that help manage large stylesheets.

Common considerations include maintaining readability and performance, minimizing reflows and repaints, and ensuring accessibility through sufficient color contrast and predictable focus styles. Good practices include avoiding overly specific selectors, documenting styles, and separating concerns to facilitate maintenance.