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Re: Shadow tree style isolation primitive

From: Ryosuke Niwa <rniwa@apple.com>
Date: Mon, 12 Jan 2015 15:51:53 -0800
Cc: Anne van Kesteren <annevk@annevk.nl>, "www-style@w3.org" <www-style@w3.org>, WebApps WG <public-webapps@w3.org>
Message-id: <C7A0665E-C5C4-463F-9F9C-0027EE6A2D77@apple.com>
To: "Tab Atkins Jr." <jackalmage@gmail.com>

> On Jan 12, 2015, at 3:51 PM, Ryosuke Niwa <rniwa@apple.com> wrote:
> 
> 
>> On Jan 12, 2015, at 2:41 PM, Tab Atkins Jr. <jackalmage@gmail.com> wrote:
>> 
>> On Mon, Jan 12, 2015 at 2:14 PM, Ryosuke Niwa <rniwa@apple.com> wrote:
>>>> On Jan 12, 2015, at 1:28 PM, Tab Atkins Jr. <jackalmage@gmail.com> wrote:
>>>> Let's assume we did it, though.  We'd have to have some mechanism for
>>>> defining an isolation boundary, and denoting whether rules were
>>>> "inside" or "outside" the boundary.  This sounds like an at-rule,
>>>> like:
>>>> 
>>>> @isolate .example {
>>>> h1 { ... }
>>>> div { ... }
>>>> }
>>>> 
>>>> Now, a problem here is that you have a conflict between nesting
>>>> isolated things and specifying isolation.  Say you have <foo> and
>>>> <bar> elements, both of which need to be isolated. You'd think you
>>>> could just write:
>>>> 
>>>> @isolate foo {
>>>> ...
>>>> }
>>>> @isolate bar {
>>>> ...
>>>> }
>>>> 
>>>> But this won't work! If you have markup like
>>>> <foo><bar>...</bar></foo>, the <bar> there is inside the <foo>'s
>>>> isolation boundary, so the @isolate rule can't find it.  You'd need to
>>>> *also* nest the "@isolate bar" rule (and all its styling rules) within
>>>> the foo one, and vice versa.  The effect of this on *three* mutually
>>>> isolated components is, obviously, terrible; let's not even mention
>>>> trying to use multiple modules together that weren't explicitly
>>>> designed together.
>>>> 
>>>> Alternately, say that it does work - the @isolate selector pierces
>>>> through isolation boundaries.  Then you're still screwed, because if
>>>> the outer page wants to isolate .example blocks, but within your
>>>> component you use .example normally, without any isolation, whoops!
>>>> Suddenly your .example blocks are isolated, too, and getting weird
>>>> styles applied to them, while your own styles break since they can't
>>>> cross the unexpected boundary.
>>> 
>>> Another alternative.  We can add a host language dependent mechanism such as an element or an attribute to "end" the current isolation, just like insertion points in a shadow DOM would.
>>> Better yet, we can provide this mechanism in CSS. e.g.
>>> 
>>> @isolate foo integrates(bar) {
>>> ...
>>> }
>>> 
>>> @isolate bar {
>>> ...
>>> }
>>> 
>>> (I'm not proposing this exact syntax. We can certainly do better.)
>> 
>> Yeah, something like that would work, but it also means you need to
>> account for all the things that might want to be isolated in your
>> component.  That's relatively clumsy.
> 
> Examples?  Are you talking about DOM APIs such as querySelectorAll and alike?  Then, please refer to my other reply [1] in which I listed use cases that involve no author scripts.
> 
>>>> This last one, though, is pretty much exactly Custom Elements, just
>>>> with the children staying in the light tree rather than being moved
>>>> into a shadow tree.  But keeping them in the light tree has
>>>> complications; it means that everything in the platform needs to be
>>>> made aware of the isolation boundary.  Should qSA respect the
>>>> isolation boundaries or not?  Depends on what you're using it for.
>>>> What about things that aren't CSS at all, like getElementsByTagName()?
>>>> That's equivalent to a qSA with the same argument, but it's not a
>>>> "selector", per se.  Manual tree-walking would also need to be made
>>>> aware of this, or else you might accidentally descend into something
>>>> that wants isolation.  Shadow DOM at least gives an answer to all of
>>>> these, by putting the elements in a separate tree.  You don't need to
>>>> think of every one individually, or deal with inconsistent design when
>>>> someone forgets to spec their new tree-searching thing to respect the
>>>> boundary.
>>> 
>>> Let's not conflate style isolation with isolation of DOM subtrees.  They're two distinct features.  Even though I do agree it might be desirable to have both in many important use cases, there are use cases in which we don't need subtree isolations.
>> 
>> I'm not trying to, I'm pointing out that "style isolation", as a
>> concept, seamlessly blends into "DOM isolation" as you move across API surfaces.
> 
> I don't see any connection between the two.  Many of the use cases I listed [1] require us to have DOM isolations.

doesn't require us to have DOM isolations.

> 
> Now, I agree there are use cases in which such DOM isolation mechanisms are desirable.  If we didn't want to add two separate mechanisms to address both use cases, we could use a host language dependent mechanism such as a dedicated HTML attribute to define a boundary.
> 
> [1] https://lists.w3.org/Archives/Public/public-webapps/2015JanMar/0112.html
> 
> - R. Niwa
> 
Received on Monday, 12 January 2015 23:52:21 UTC

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