DOCTYPE

I recommend the following XHTML DOCTYPE:

It uses causes a document to be validated using XHTML.
It uses transitional XHTML, which allows a few essential attributes, such as the start attribute in <ol> and the value attribute in <li>.
It places browsers into almost-standards mode, which is essential for styling in CSS.

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

I recommend the following two HTML DOCTYPEs if you prefer HTML syntax.

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

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd">