Cascading Style Sheets (CSS)

Cascading Style Sheets (CSS)Cascading Style Sheets (CSS) is the language used to define the layout and design of web pages. CSS controls colours, fonts, spacing and arrangement, among other things.Open glossary entry → separates the design of a web page from its content structure in HyperText Markup Language (HTML)HyperText Markup Language (HTML) is the markup language for web pages. It defines the structure and content of a page, such as headings, paragraphs, links and images.Open glossary entry →. While HTML defines what is on a page, CSS determines how it looks, such as colours, fonts, spacing and responsive behaviour across devices.

For search engine optimisationSearch Engine Optimization (SEO) covers all measures that help a website become more visible in unpaid (organic) search results.Open glossary entry →, CSS is usually relevant indirectly, but important in some places:

• CSS files should not be blocked for crawlers, so Google can renderJavaScript rendering is the process by which a browser or crawler executes JavaScript code to build the final, visible page. Content that is not already in the delivered HTML only appears after this step.Open glossary entry → and assess the page correctly,
• content hidden via CSS but present in the HTML (for example in tabs) generally remains discoverable,
• layout and loading behaviour affect user experience and performance metrics.

If important CSS resources are blocked, a page may appear differently or incompletely to crawlers. Checking blocked CSS and JavaScript resources is therefore part of a good technical audit.