XHTML/CSS

XHTML combines HTML and XML together to simplify coding. CSS came around as a way of putting all the font colors, fonts, size, etc. together in one location that made it easy to change the look of your page. Web standards are the standards set by W3C in order to unify the way people code and the way pages look.

Sources:
http://webdesign.about.com/cs/xhtmlxml/a/aa013100a.htm
http://en.wikipedia.org/wiki/Cascading_Style_Sheets
http://webstandardsgroup.org/standards/