CSS

Cascading Style Sheets -- it is a style describing language that writes down the appearance of HTML and XHTML documents. You can use it to describe the style of other XML documents such as SVG and XUL. It is a text based file which has a .css attribute.

With the help of the CSS you can format HTML documents independently of the content. You can assign different looks to specific documents: color font-type, heading, box. You don't have to write these in the HTML document again and again, you just have to apply a stylesheet, CSS tag. That way you can create logical, clearer, smaller HTML codes. The W3C standard even requires to use CSS for the old HTML presentation tags. More: http://en.wikipedia.org/wiki/CSS