Define all focus events

[annevk]

We should at least define

• focusin
• focusout

and maybe

• DOMFocusIn
• DOMFocusOut

if they are still implemented in most browsers.

uievents/order-of-events/focus-events/ has some tests.

w3c/uievents#88 has a discussion by @dtapuska @smaug---- et al about changing existing implementations to be more useful. It seems that needs to be resolved one way or another before we take action here.

I also somewhat feel it'd be more logical for HTML to define all the bits related to focus events, such as FocusEvent, since it already defines the dispatching logic. It doesn't make much sense to me for that to be defined in https://w3c.github.io/uievents/#events-focusevent and results in fragmented discussion about how these events ought to work.

(This supersedes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25897.)

[cvrebert]

As of 2 years ago, DOMFocusIn & DOMFocusOut were only in Blink and WebKit.

[AmeliaBR]

If these get added to the spec, is it safe to assume they will be added to the GlobalEventHandlers mixin?

(That makes it easier for SVG, to just defer standardization to HTML.)

[cvrebert]

DOMFocusIn/DOMFocusOut usage stats for Chrome:
https://chromestatus.com/metrics/feature/timeline/popularity/211

[josepharhar]

I created a PR to define focusin and focusout: #10235