CSS

Cascading Style Sheets control the presentation (that is the appearance and formatting) of your pages, and easily make site-wide presentation changes. The purpose to CSS was to remove the styling from the content of the site. CSS can control positioning, layout, font, colors and style information.

The simple separation of style from content has had tremendous implications to the web. It has allowed browsers the ability to cache and reuse styling across servers; it has given developers the flexibility to program a web app and let the designer design the site simultaneously, without the need to wait for the other; it has allowed designers to reuse their design work for basic elements (increasing the abstraction of these elements). In layman’s terms: it has allowed for more content to be posted online, and delivered it faster to the end user.

The end users are not only people but also search engines that crawl through the websites. If the CSS and HTML on a website is not written correctly, it could stop your website from being indexed and ranked correctly.