Cascading Style Sheets also known as css is something that every coder needs to learn. they give a different feel and look to your XHTML site. essentially its a file that has been created by the coder, and what the coder can do with this is attach it to any of his or her DTD to give each one the same theme or color scheme or how you just want the page to be displayed. Html is the backbone for the world wide web. but with things changing the way that they have html just cant do everything that we want it to. So XHTML was created. XHTML is an upgrade from html. Aswell during the html period most coders would create extremely sloppy coding in their sites . Now the websites would still work but with this sloppy html coding it would give the web browser problems in some cases. But with XHTML things must be exact and its a very strict way of creating a website. but the end result is that you will have a better looking site AND you will be able to CSS to the site to make it look better, but css can be used with regular html.

Web standards were created for people who designed web sites. the web standard is there so that web browser creators will know what they want their browser to display.

A great place to get started if your interested in it would be to go to (here)