The !DOCTYPE directive is one of the most commonly misunderstood parts of markup in the entire page, additionally, most WYSIWYG editors get it wrong.
This serves two major purposes, with one common goal – ‘standards’!
1. Validators can easily look to the markup to determine which version of the HTML/XHTML standard to validate against (actually, it’s done against the DTD that you define within the tag).
2. Browsers also use this tag to determine which version of the HTML/XHTML standard to render with… however, the most common browser currently on the market (MSIE), chooses to ignore it!
3. Other markup languages (decendent of SGML, like HTML and XHTML) like WML also use this tag.
1. This tag doesn’t follow standard XML rules, there is NO close tag, but it’s not self-closing either.
2. This MUST be the very first line of output in your HTML.
3. Because of the MSIE issue, you cannot start your page with:
<?xml version="1.0"?> for XML!
Some common examples:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">