Validate Your HTML Files as HTML 4.01

An HTML document is validated against a Document Type Definition (DTD).
Before an HTML file can be properly validated, a correct DTD must be
added as the first line of the file.

The HTML 4.01 Strict DTD includes elements and attributes that
have not been deprecated or do not appear in framesets:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

The HTML 4.01 Transitional DTD includes everything in the
strict DTD plus deprecated elements and attributes:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

The HTML 4.01 Frameset DTD includes everything in the
transitional DTD plus frames as well:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">

Dreamweaver uses this Document Type Definition:

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