[Bug 19684] [Shadow]: shadow reference combinator should be css function.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=19684

--- Comment #2 from Takashi Sakamoto <tasak@google.com> ---
(In reply to comment #1)
> (In reply to comment #0)
> > I think, /select/ is a little confusing for web developers. It would be
> > better to use css function instead, e.g.
> > - distributed-into(), or
> > - selected-by()
> > 
> > I will write the reason why I suggest css function.
> > Current shadow dom spec says, 
> > 
> > - The combinator value must be select
> > - The first compound selector of the combinator must match an insertion point
> > - The second compound selector must match an element, distributed to this
> > insertion point
> > 
> > Its example is .some-insertion-point /select/ div.special.
> > 
> > However, any element can have "select". For example, users can write '<div 
> > class="some-insertion-point" select="...">'.
> > 
> > So it is difficult to know whether /select/ is just reference combinator or
> > shadow reference combinator.
> 
> "select" is not a valid attribute for a "div".  I don't see what is
> difficult.

Would you mean whether reference combinator works or not depends on whether the
attribute is valid or not?

I looked at the reference combinator's spec and couldn't found that attribute
selector only works for valid attribute or something. I could find the
following:

"attribute selectors must be considered to match an element if that element has
an attribute that matches the attribute represented by the attribute selector."

> > For example,
> > <div class="shadow-host">
> >    <#shadow-root>
> >       <style>
> >       .some-insertion-point /select/ div.special { color: red; }
> >       </style>
> >       <div class=".some-insertion-point" select="#B">
> >       <content class=".some-insertion-point">
> >       <div id="B">This text is red?</div>
> >    </#shadow-root>
> >    <div class="special">This text should be red!</div>
> > </div>
> > 
> > The ".some-insertion-point /select/ div.special { color: red; }" should be
> > applied to distributed node or just div in shadow dom subtree? Both?
> > 
> > The most difficult thing is that we cannot decide whether
> > ".some-insertion-point" points really insertion points or not when parsing
> > the rule.
> > 
> > So by using css function, we can write:
> > - div.special:distributed-into('.some-insertion-point') { color: red; }, or
> > - div.special:selected-by('.some-insertion-point') { color: red; }
> > 
> > This is very easy to know that the style should be applied to distributed
> > nodes.
> > 
> > What do you think about this?
> 
> I think the example you brought up is rather weak. I honestly don't see a
> problem with this. Thus, in my opinion, this is bike-shedding.

If shadow reference combinator doesn't cross any treescope boundaries, I agree
with you that this is not a problem.
However, the current spec says that sometimes reference combinator can cross
treescope boundary. I feel that it might be inconsistent.

> Also, you need to argue with Tab and the CSS WG about this. He is the one
> who recommended to use reference combinator.
> 
> Also, here's more information on the reference combinator:
> http://www.w3.org/TR/selectors4/#idref-combinators

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