Update 5.4.3 (tabindex attribute) with Shadow DOM info

[LJWatson]

Focus management with respect to tabindex in the shadow DOM was discussed at TPAC [1]. An issue to update the Shadow DOM spec has since been opened [2].

The issue includes a suggestion to update the HTML specification (5.4.3 the tabindex attribute [3]).

[1] https://www.w3.org/2015/10/26-webapps-minutes.html
[2] WICG/webcomponents#375 (comment)
[3] http://w3c.github.io/html/editing.html#the-tabindex-attribute

[chaals]

This might be ready for inclusion now.

[siusin]

We're closing this issue on the W3C HTML specification because the W3C and WHATWG are now working together on HTML, and all issues are being discussed on the WHATWG repository.

If you have questions about this, please open an issue on the W3C HTML WG repository or send an email to public-html@w3.org.