Make <menuitem> parse like unknown elements #2308
Description
Previous issue:
#234
The change to make <menuitem> parse more like <option> (with optional end tag) has not been adopted by any browser engine, as far as I know, and @hsivonen says we should change the standard to make it parse like unknown elements instead.
cc @sanjoypal @inikulin @wycats @bzbarsky
From @hsivonen in #2253 (comment)
On a general level, I'm worried on security grounds about keeping changing the HTML parsing algorithm. If the server and the browser implement a different spec snapshot (libraries deployed in server-side software don't update every six weeks even if their upstream repos did), the server can parse something and decide it's OK and the browser may parse it into something different. (Depending on the parser change, this might apply even if the server parses and serializes, which the server should do.)