Sanitizer: Be aware of elements that have different parsing rules

[leafo]

There are some html elements (eg. iframe) which use different parsing rules for the content of the tag. The sanitizer should be aware of this and treat those tags separately. None of the elements in the default whitelist are currently affected by this.

Note: Also worth investigating other elements that have special parsing rules: https://html.spec.whatwg.org/multipage/parsing.html#special

See: https://html.spec.whatwg.org/multipage/parsing.html#html-parser

[leafo]

Random, but enjoying this line from the HTML spec: