Re: [webcomponents] Introduce "delegatesFocus" flag to shadow root (#126)

Thus, the changes I would like to introduce is basically listed [here](https://github.com/TakayoshiKochi/tabindex-focus-navigation-explainer/blob/master/TabindexFocusNavigationExplainer.md#delegatesfocus-details),
by giving `delegatesFocus: true` in `attachShadow()` will affect:

1. if shadow host has 0 or positive `tabindex` attribute value, the sequential navigation order honors that value, but actual focus goes into an element in its shadow (depending on navigation order, first or last focusable element in shadow will be picked)
2. `focus()` on a shadow host will also delegate focus to its shadow
3. `autofocus` attribute on a shadow host will also delegate focus to its shadow
4. If non-focusable area in shadow is clicked, shadow host gets focus but then delegates focus to its shadow
5. CSS `:focus` pseudo class on shadow host should match when its shadow or its descendant has focus

@rniwa @travisleithead @annevk any opinion for each item above?

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/126#issuecomment-188160444