The <iframe> tag is invalid in XHTML strict AND HTML strict
BUT.....
Apparently it is valid in HTML5
Making a comeback?
I think its important to distinguish between <iframe> and
the nasty old school <frame> tag.
<iframes> are like embedded objects within a page that obey normal
formatting rules.
The <frame> tag defines one particular window (frame) within a frameset.
Each frame in a frameset can have different attributes, such as border, scrolling, the ability to resize, etc.
To be valid it also requires a Frameset DOCTYPE.