Intent to Implement and Ship: HTMLTextAreaElement.autocomplete

[Matthew N.]

*Summary*:
Allows web developers to get the effective/normalized value of the
autocomplete attribute in the same way they currently can for <input> and
<select>.​

Example: <textarea autocomplete=" On "></textarea> returns "on" for
textarea.autocomplete.
The return value will respect the pref dom.forms.autocomplete.formautofill,
like we already do for <input> and <select> meaning it will only return
"on"/"off" if Form Autofill isn't available to support the other values.

*Bug*: https://bugzilla.mozilla.org/show_bug.cgi?id=1020698
*Link to standard*:
https://html.spec.whatwg.org/#the-textarea-element:attr-fe-autocomplete
*Platform coverage*: All but iOS
*Estimated or target release*: Firefox 59
*Preference behind which this will be implemented*: None (it's simply
exposing shared logic for <select> and <input> on <textarea>
*Is this feature enabled by default in sandboxed iframes*
​:​
Yes
*DevTools bug*: N/A since it's just a
​fairly basic DOM property. In theory devtools could use the internal
getAutocompleteInfo API to show the browsers' interpretation of the parsed
attribute.
*Do other browser engines implement this*
​:​
According to https://wpt.fyi/html/dom/interfaces.html's
"HTMLTextAreaElement interface: attribute autocomplete" row:
* Safari: Shipped <https://bugs.webkit.org/show_bug.cgi?id=150731>
​ in unknown version <https://bugs.webkit.org/show_bug.cgi?id=150731>
* Chrome: N
​ot yet
<https://bugs.chromium.org/p/chromium/issues/detail?id=758074>
​​
​ <https://bugs.chromium.org/p/chromium/issues/detail?id=758074>, but​
​i​
nten
​ding​
to
​i​
mplement and
​s​
hip
<https://groups.google.com/a/chromium.org/d/topic/blink-dev/2tutRHmhwTE/discussion>
* Edge: No
*Tests -*
*​*
* Is this feature fully tested by web-platform-tests*
​:​
​ https://wpt.fyi/html/dom/interfaces.html has basic interface tests and ​
https://wpt.fyi/html/semantics/forms/the-form-element/form-autocomplete.html
tests the common parsing logic (not specific to <textarea>)

​Thanks,
Matthew N. (:MattN)