Cascading Style Sheets (CSS) is a style sheet language used for describing the look and formatting of a document written in a markup language. While most often used to style web pages and interfaces written in HTML and XHTML, the language can be applied to any kind of XML document, including plain XML, SVG and XUL. CSS is a cornerstone specification of the web and almost all web pages use CSS style sheets to describe their presentation. (Source: Wikipedia)
The amount of mobile devices is growing and therefore mobile internet browsing is growing too. Usually connection to internet is somewhat slower on a mobile device and in addition it might cost quite much to download a large bunch of data. So at least the mobile users would be quite happy if the page loads quickly and does not contain that much data.