Specify expected behavior for when users change input field directionality

[zcorpan]

The spec infers but doesn't say explicitly that users can change the directionality of input fields. For the dirname attribute to properly submit the user-set directionality, this action needs to set the dir attribute on the element. Session restore also seems to assume in a note that that is how it should work.

Chromium, WebKit and Gecko allow users to change directionality and all set the dir attribute when doing so, at least for (some) input and textarea elements. See https://www.w3.org/International/questions/qa-html-dir#userexplicit (contrary to what is currently stated there, Firefox does set dir: w3c/i18n-drafts#469).

For contenteditable it's not interoperable: Gecko sets dir on the body element (bug 962015), Chromium sets style="direction: rtl;" on all paragraphs in the editable host (inserting a div if neccessary), WebKit sets style="direction: rtl;" on the selected paragraph (inserting a div if neccessary). I think Gecko's behavior is not great since it affects the entire page, not just the editable area. But using dir seems better than style. I suppose per-paragraph directionality is better than per editing host.

cc @whatwg/forms @vinhill

[annevk]

cc @rniwa