Ignore title attributes on <link rel=stylesheet> in shadow trees

[hayatoito]

In #530, we have not discussed anything about how title= attribute should be treated in <link rel="stylesheet">. Yeah, that's our mistake. :)

I would like to have a consensus on the behavior here before proceeding this code review further.

My proposal is that we ignore title= attribute completely, as we did that for <style> elements in #391.

Note that the difference between <style> and <link> here is that <link> can specify alternative stylesheet by rel=alternate. See https://developer.mozilla.org/en-US/docs/Web/CSS/Alternative_style_sheets for general information about alternative style sheets.

The proposal is effectively making:

1)  <link rel="stylesheet" title="x">
2)  <link rel="alternate stylesheet" title="x">

behave as:

1)  <link rel="stylesheet"> // always enabled
2)  <link rel="alternate stylesheet"> // never enabled

See https://codereview.chromium.org/2177163002#msg52 for the rationale. I agree with @rune-opera 's rational there.

[domenic]

I spent some time learning about this and agree with @hayatoito and @rune-opera that we should ignore title attributes for stylesheet links inside a shadow tree. The reasoning is basically the same as was decided (before my time, in #391) for style elements. The title attribute has a global affect on the document, both in terms of adding to the list of named stylesheets and in terms of changing some defaults and preventing some other stylesheets from applying. It would be better to avoid this global affect from components that are supposed to be encapsulated.

I put together https://ns-jmzlbrrfkc.now.sh/ as a demo page for anyone else who would like help understanding how the title attribute works in combination with alternate. In Firefox you can switch between style sheets using the menu, View, Page Style.

I hope we can get consensus on this quickly (@smaug----, @travisleithead, @rniwa). It might be a bit tricky to spec rigorously until we first straighten out whatwg/html#848, but if everyone is agreed we can record consensus here or maybe just with a non-rigorous line in the shadow DOM spec.

[rniwa]

Yeah, ignoring title attribute here seems sensible to me.

[smaug----]

yes, ignoring sounds good.

[zcorpan]

I suppose when HTML says to "Create a CSS style sheet" (for both link and style) we just don't pass anything in for the "title" if it's in a shadow tree, right?

[annevk]

Yeah, that is probably the easiest. And we should test that StyleSheet.prototype.title returns null.

[hayatoito]

Sounds good to me because StyleSheet.prototype.title is a read only attribute.