Note: {margin: auto;} doesn't work in IE5 quirks mode. Centering with an extra div works, but computers still using quirks mode must be very small now, and I think it should soon be able to be ignored.

Open this page in Firefox and centering with an extra div doesn't work. You need to use margin: auto; and text-align: center; for them both to work.

Put your browser into "quirks" mode by deleting the Doc statement, or include a comment preceding the Doc statement. You'll need a combination of all 3 to work in every browser, but as previously stated, I wouldn't worry too much about quirks mode, unless it's something important you're centering.