Intent to prototype: autocapitalize

[Makoto Kato]

autocapitalize attribute is a html attribute that control
capitalization of software keyboard.

Originally, although this was implemented by Safari/iOS 5 and B2G had
the same feature in a part of inputmode (bug 796544 [*1]),
standaralized version became the autocapitalize attribute. Early
implementation of this attribute on Safari/iOS was <input> element and
<textarea> element only. The latest WebKit and Blink are available for
contenteditable too and WHATWG spec also considers contenteditable.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1425291

Platform coverage: All. But Windows and macOS have no API to control
capitalization of software keyboard.

Preference: This is enabled/disabled via dom.forms.autocapitalize

Devtools support: N/A. DevTools doesn't have software keyboard emulation.

Other browsers: Yes, Chrome and Safari (iOS only)

web-platform-tests:
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/html/editing/editing-0/autocapitalization/autocapitalize.html

Restricted to secure contexts: No

-- Makoto

*1 https://bugzilla.mozilla.org/show_bug.cgi?id=796544