Is <slot> a replaced element or not?

[haoqunjiang]

Per the WHATWG HTML Spec, the <slot> element is listed in the section "Non-replaced elements". However, given its presentation hint (display: contents), it seems that <slot> might better be treated as a replaced element. Also Chrome 53+ seems have implemented it as such (::before and ::after selectors no longer work on <slot>)

[annevk]

There's disagreement between WebKit and Blink as to what is correct here. @rniwa et al feel quite strongly ::before and such should work.

[rniwa]

I think whether we treat slot as a replaced element is a slightly orthogonal issue to whether it has display: contents as the style or not. Having said that, slot is kind of special since its content is the slotted nodes so I could see it being treated as a replaced element. Really though, slot is a special element different from replaced or non-replaced element.

[hayatoito]

When users specify an author rule, slot { display: block }, I would like <slot> to behave like a <div>, in terms of layout.
Though other nodes are assigned to children of a slot in the layout tree, there should be no difference between <slot> and <div>, other than that, in this case.

Does it mean <slot> is NOT a replaced element?

I do not fully understand the impact of letting it being a replaced element. What is a user visible impact?
::before (or ::after) can not be applied for a replaced element, per the spec?

[haoqunjiang]

@hayatoito It's stated in CSS Generated Content Module Level 3:

Replaced elements do not have '::before' and '::after' pseudo-elements

[hayatoito]

Thanks! Then, I propose the status quo; <slot> should be a Non-replaced element.

Regarding the current behavior in Blink, please do not take it seriously. No explicit intention there.
The current support of <slot> in Blink is incomplete because Blink does not support "slot" in a flat tree.
See #308 for details. Blink does not create any layout box for a slot, as of now.

[hayatoito]

Let me close this with status quo since this issue did not get the attention.