xhtml

The W3C has made alot of efforts to standardise markup language on the web.The reason behind this is to have 'cross platform interoperability'.......in short, to have websites looking and acting the same across many various types of computers, browsers and Internet devices. In applying this goal to html, xhtml was born.
The good news for the customer is a slicker, faster loading site (because there is less code and the browser is able to read it easier ).

css

css has two great functions.......you can separate style (color of text etc) from the structure and layout of the site.This means a very easy method to change colors throughout the site, you change one css file and all html files are changed.
Another great benefit using css styles for layout of your pages.The old fashioned way was to use tables which were slow and unreliable for layout.Tables should only be used for holding tabular data. Because of improvements to browsers and the css technology, developers can now provide excellent, dynamic and fastloading sites.

my sites are designed using xhtml + css