Cascading style sheets (CSS) is a style sheet language used to set out the way in which a website is visually presented online, for example with the use of headers, footers, and navigational indicators. A website or document using CSS will be written in a markup language such as HTML, XHTML, plain XML, SVG or XUL. 

CSS follows a predictable prioritising scheme, according to whether one or more style rules are applicable to the elements of the webpage. By using CSS, a web developer can successfully separate the way in which the content and presentation of a website are handled. 

Presentation aspects such as choice of font, colours, and overall layout of web pages can be controlled across hundreds or even thousands of pages using CSS. 

CSS can heavily influence factors such as accessibility (some colour contrasts may prove hard to read by visually impaired or colour blind users, for example), as well as impact on the performance of the website by improving page loading speeds.

CSS is important in responsive website design. This is when a single has many CSS rules that can adapt the presentation of it according to the device it’s being viewed on.


When it comes to SEO, CSS has many compelling benefits. Modern search engines mark websites against an increasingly complex range of criteria, and using CSS can help a website to perform better in several key areas, by standardising design and layout elements. 

In terms of page loading time, using CSS can help to reduce complexity and thereby boost page speeds by keeping all of the information relating to the webpage’s presentation in a single CSS file. This allows search engine crawlers to easily find the information they need in order to rank a website highly for appearance. CSS can also be used to emphasise relevant keywords without stuffing the content (a practice which can now negatively affect a website’s rankings). 

Perhaps most importantly though, CSS makes updating a large website with many pages quick and simple.