Allow lowercase <iframe> tags for XHTML complience

In HTML `element.tagName` is returned in upper-case, but in XHTML it is returned [as it is defined in the document](https://developer.mozilla.org/en-US/docs/Web/API/Element.tagName#Notes).

As XHTML pages are required to have their element names in lower case, as per the [W3C XHTML Spec](http://www.w3.org/TR/xhtml1/#h-4.2), the current code doesn't work in XHTML.


I'm not being pedantic and I don't mean to enforce a style choice either, it's just that browsers do not recognise the tag <IFRAME> when the page is served as XHTML.