Make <slot>'s UA style, "display: contents", be !important

[hayatoito]

Originally, discussed at https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/5s1nbytTL68/Kx-dtQ34BAAJ.

It would be better to make <slot>'s UA style, "display: contents", be !important to prevent accidental overwrite by author rules.

slot {
  display: contents;
}

is defined here: https://html.spec.whatwg.org/multipage/rendering.html#flow-content-3

[domenic]

@rniwa, what do you think? You were the original display: contents proponent, if I recall.

[rniwa]

I don't think we should do this. Otherwise author won't be able to override display of a slot. Being able to stylize slot as a different display type servers valid use cases.

[hayatoito]

I see. I don't have a strong opinion on this.

@rniwa, I don't have any data yet, but if Blink has a trouble in making slots participate in a flat tree because of this rule, are you okay to make this rule be !important?

I hope that Blink wouldn't have such a trouble, but just in case.

[rniwa]

No. We don't want to make this !important. It's okay if some engines had bugs.

[hayatoito]

Okay. Let's close this issue.

I might re-open this issue if Blink encounters a trouble because interoperability is much more important than use cases which we have not found.