740856 - Support pseudoelements syntax in xbl

Status: RESOLVED WONTFIX

(Core :: XBL, defect)

Description

[Neil Deakin]

XBL 2 has a pseudo attribute to allow specific anonymous content nodes to match as a pseudo element.

For example:

<xbl:content>
  <image src="hello.png" xbl:pseudo="icon"/>
</xbl:content>

would allow:

element::icon {
  border: 1px solid red;
}

Comment 1

[Daniel Glazman (:glazou) (not active in Mozilla any more)]

I think this is bad design. The proposed CSS pseudo-element should probably
be a functional notation. For example:

  ::xbl-content(icon)

That allows a lighter impact on the CSS parser and a greater extensibility.

Comment 2

[Boris Zbarsky [:bzbarsky]]

Indeed.  For example, implementing the proposal in comment 0 would require a pretty complete rewrite of how pseudo-elements are handled in the style system in Gecko.

Comment 3

[Neil Deakin]

I'm assuming that the concern above only applies when extended to arbitrary pseudo elements. Note that xbl2 only defines five specific pseudoelements that are allowed:
  value, choices, label, repeat-item, and icon

That said, custom pseudo elements could also be supported via a different syntax, something akin to the one described in comment 1, since there will likely be value of it in toolkit bindings.

Comment 4

[Boris Zbarsky [:bzbarsky]]

If the set of possible pseudo-elements is fixed and not too big, then comment 2 doesn't apply, yes.

Comment 5

[Anne (:annevk)]

Web Components \o/