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) 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 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 JavaScript 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.