IE6 in quirks mode

Is Internet Explorer running in quirks mode or standards mode?

It turns out there is a very simple way to tell. Just add in your address bar:

javascript:alert(document.compatMode)

If the browser responds with: "CSS1Compat, it is in standards mode. If it responds with "BackCompat", it runs in quirks mode. See this link for more details.

In the latter case, you probably want to change your DOCTYPE, for instance to:

http://www.w3.org/TR/html4/loose.dtd">

See this link for a useful list of DOCTYPEs