<!DOCTYPE>

Defines the document type, this tells your browser which specification your document uses. No attributes or css values to apply, just be sure it includes it complete web address.

Legend

These give hints, nothing more. Their presence is explained at tip collection for this tag:

XRequired attribute in (x)html
Required attribute in (x)html. Use it.
AAccessibility requirement
Required and/or recommended attribute or value for an accessible site. Use it.
?Alert!
This attribute or value has issues. Check tip page for reason and alternatives.
‼Cool!
Just cool; real enhancement when used here.
html attributes for <!DOCTYPE>
css properties applicable to <!DOCTYPE>

Clip function works in ? (Its been a while, it probably doesn't work any more).