support for picture and other harmless elements

[lewisje]

I realize that picture is not in HTML5 but rather was proposed after it was finished, but it is implemented in many browsers, it is on a standards track, and it is as harmless as the img and source elements it's based around.

I also wonder what you think about allowing a couple old and nonstandard but still harmless elements: blink and marquee (even though I am aware that no modern browser even supports blink anymore).