Re: [WICG/webcomponents] "open-stylable" Shadow Roots (#909)

> The original idea (this.attachShadow({mode: 'open-stylable'});) would cause FOUC.
> 
> Why is that necessarily the case? This is a script necessary for rendering. Loading such a script after rendering will casuse FOUC, loading it before will not.
> 
> This would also work with declarative shadow DOM which does not require script and is therefore not likely to FOUC.
> 
> `<div>`
>   `<template shadowrootmode="open-stylable">...</template>`
> `</div>`

attachShadow() is not necessary in the first place for rendering a declarative shadow tree; even a Custom Element can have a shadow root before it gets upgraded. 

That's why declarative shadow DOM works for server side rendering while also providing built in opt-in (in the shadow layers proposal) for Custom Element authors. And is polyfillable.

So no new mode is needed for either standalone shadow trees (or Custom Elements that opt-in).  To me, FOUC-free is both necessary and achievable.

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