It is unclear how title attribute should work in Shadow DOM

[smaug----]

https://html.spec.whatwg.org/#attr-title doesn't hint anything about crossing shadow boundary, but seems like at least blink does look for title attribute also in light DOM when mouse is over shadow DOM.

[kojiishi]

Please also refer to Jun 29 comment on #3699.

[smaug----]

http://mozilla.pettay.fi/moztests/title.html

[annevk]

So this would be replacing "nearest ancestor" with "nearest composed ancestor" and defining "composed ancestor" in similar terms to what "get the parent" does for events.

[rniwa]

TPAC F2F consensus: Spec what all browsers have implemented with the semantics @annevk mentioned above.