Standardize UA styles / presentational hints for embed hidden

[nt1m]

We noticed this WPT: https://wpt.fyi/results/html/semantics/embedded-content/the-embed-element/embed-hidden-attribute.html?label=master&label=experimental&aligned

(which looks perfectly reasonable), does not have a spec counterpart to it.

CC @mfreed7 who created the test.

[nt1m]

cc @whatwg/css

[mfreed7]

Thanks for raising this! It came out of this Chromium bug, which has more discussion, particularly about cross-browser behavior.

Can you comment on why the hidden attribute (which is global) doesn't just automatically apply to <embed> elements? It says

When an element has the hidden attribute in the hidden state, it indicates that the element is not yet, or is no longer, directly relevant to the page's current state, or that it is being used to declare content to be reused by other parts of the page as opposed to being directly accessed by the user. User agents should not render elements that are in the hidden state. This requirement may be implemented indirectly through the style layer. For example, a web browser could implement these requirements using the rules suggested in the Rendering section.

I was just going on that when I wrote the test.

[nt1m]

I agree about the hidden attribute applying, I was more referring to the width: 0; height: 0; part of how browser engines are implementing hiding.

[mfreed7]

I agree about the hidden attribute applying, I was more referring to the width: 0; height: 0; part of how browser engines are implementing hiding.

Ahh, I see. You want to spec more of the detail of how "should not render" is achieved?

[nt1m]

Yeah, I received a PR for a change regarding that in WebKit, and it was unclear to me what was the right way to stop rendering.

[zcorpan]

https://html.spec.whatwg.org/multipage/rendering.html#hidden-elements says

embed[hidden] { display: inline; height: 0; width: 0; } 

(display: inline overrides the [hidden]:not([hidden=until-found i]) { display: none; } rule)

Does that not cover what the test expects?

[annevk]

I think it does. @nt1m please reopen if you disagree.

[zcorpan]

Hold on. Specificity wars are at play here, right?

embed[hidden] has specificity 0.1.1.

[hidden]:not([hidden=until-found i]) has specificity 0.2.0, so it wins.

We need [hidden]:where(:not([hidden=until-found i])) to make it 0.1.0 so the display: inline will apply.

[zcorpan]

alternatively [hidden]:not([hidden=until-found i]):not(embed)