Cascading Style Sheets (CSS)

Cascading Style Sheets (CSS) is a term describing a stylesheet language used to control the presentation of a page written in HTML or other markup language.

Like other web-related standards and languages, CSS was introduced and is maintained by the World Wide Web Consortium (W3C).

CSS is mainly used to style the web pages and separate content from presentation. It has a simple syntax and allows web designers to completely change the look and feel of the whole website without touching the HTML code, e.g. creating  different "skins" to cater for to different users' preferences or clients’ branding needs.

CSS styles can be embedded in the HTML page itself, but the best way is to keep all CSS classes in a separate file or files with the .css extension. This has two benefits: it lightens the code of the HTML pages, which is important for SEO and download time, and it means that you only have to edit one file (the one containing a CSS class) to make the changes on every page that calls this particular CSS file.

When CSS is used for colours and fonts, a single change on the CSS file can change the look of every page within the same website. To achieve the same effect by changing the attributes within all the <font> tags, a webmaster would have to edit every tag separately. With big websites this work might take several days. That's why the <font> tag has been deprecated along with some other tags that control the look and feel of a webpage.

The more advanced use of CSS is when the whole layout of a webpage is being held together using CSS styles only. Historically, web designers often used HTML tables to control the layout of the page; it was easy, but it was not what the tables were initially created for and affected accessibility, page load time and even SEO (to some extent) detrimentally. So, web designers aiming at quality have lately been switching to table-free, CSS-controlled layouts.

Controlling layouts using CSS is not a trivial task, because all existing browsers read CSS a little bit differently. Working out cross-browser solutions (i.e. writing CSS classes that will allow the page to look the same in all browsers and not fall apart at different screen resolutions) is a lot of work and many webmasters just give it up. But, it has become less complicated, as browser vendors release new versions of their software and gradually improve their interpretation of CSS. One day they will probably find common ground and start reading CSS classes the same way, so any CSS-powered page will automatically look exactly the same in Internet Explorer, Opera, Firefox and Safari. Besides, the most skilled and charitable webmasters often create samples of cross-browser compatible CSS-controlled layouts and offer them for free re-use on their websites. One site containing a large and useful collection of sample layouts is Search Engine Friendly Layouts.

All this allows webmasters, who would like to offer their customers real quality services, to switch to CSS-powered layouts right now.