First of all. There are currently Three different ways to declare an xhtml document. These ways are: strict, transitional and frameset.
Strict: This code below should appear at the top of the page before <html> or anything at all. If you do not include it, it will be included automatically as html 4.01 in most cases. Use it when you want a super clean code, Cascading Style Sheets (CSS) should be used with it.
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
Transitional : Displaying style's in the current xhtml file are valid. Cascading style sheets are not needed to declare the style of the website. This will work in all browsers. (Some browsers dont support css). It's rare to come across a browser that doesn't support css. If your browser doesn't support css, update it! Life goes on.
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Frameset : Use this when you have one or more frames in the xhtml file.
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
More HTML and CSS: 
