Webmaster Forum / HTML, CSS, Scripts / CSS / March 2005

  <iframe> is not a valid element in XHTML strict.
  Read the error messages when you validate a document. Excerpt from the
error message of the W3C validator:

"You have used the element named above in your document, but the document
type you are using does not define an element of that name. This error is
often caused by incorrect use of the 'Strict' document type with a
document that uses frames (e.g. you must use the 'Frameset' document type
to get the '<frameset>' element), or by using vendor proprietary
extensions such as '<spacer>' or '<marquee>' (this is usually fixed by
using CSS to achieve the desired effect instead)."