[css-pseudo] Need a way to styling the disclosure triangle of the <details> (or <summary>) element

This question was raised in bug 1221416
<https://bugzilla.mozilla.org/show_bug.cgi?id=1221416> when implementing
<details> and <summary> elements in Firefox.

We might need a way to support styling for the disclosure triangle of the
<details> (or <summary>) element. Currently, webkit supports a non-standard
pseudo-class summary::-webkit-details-marker as this article
<http://blog.teamtreehouse.com/use-details-summary-elements> said.

Xidorn proposed here
<https://bugzilla.mozilla.org/show_bug.cgi?id=1221416#c2> that we could add
::marker support for <summary> element in pseudo-element spec
<https://drafts.csswg.org/css-pseudo-4/#marker-pseudo>.

Any ideas?

Ting-Yu