Iframe elements without <title> in <head> cause failures for 2.4.2 incorrectly #286

The sniff for 2.4.2 checks all elements in the document without understanding the context. Currently it will cause a failure if the element does not contain a title but according to https://www.w3.org/WAI/WCAG21/Techniques/html/H64 an iframe document without a <title> is permitted if the iframe has a title attribute.