Web Design in a Nutshell, 3rd Edition

Use a DOCTYPE Declaration

Every HTML or XHTML document should begin with a DOCTYPE declaration that tells the browser which language your document was written in. An example of a DOCTYPE declaration for a document written in strict XHTML 1.0 looks like this:

<! DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

Not only is it the correct thing to do according to the W3C, but current browsers have the ability to switch into different rendering modes (e.g., “Standards” “Almost standards,” “transitional,” and “quirks”) based on the DOCTYPE. Omitting the DOCTYPE may adversely affect the way your page renders in the browser. Available DOCTYPE declarations and DOCTYPE switching are discussed in more detail in Chapter 9.