Summary: 
Two CSS properties scrollbar-{face,track}-color to style scrollbars in a color 
by author.

These properties intends to provide a more restricted feature set for 
controlling scrollbar styling than ::-webkit-scrollbar-* pseudo-elements 
shipped in WebKit and Blink, so that authors can get native scrollbars while 
still have some styling fitting their pages. 

There is a consensus in the CSS working group that the existing pseudo-element 
API is too powerful, and since scrollbar is an evolving technology, we want to 
provide a restricted API to give users a native experience while allowing 
authors to customize to some extent.

Hopefully with these properties (and one another controlling scrollbar width or 
style to fulfill thin scrollbar usecases), WebKit and Blink would be able to 
unship their current pseudo-elements, so that we wouldn't need to implement 
them to get web compatible.


Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1460456

Link to standard: 
https://drafts.csswg.org/css-scrollbars-1/#scrollbar-color-properties

Platform coverage: Desktop

Estimated or target release: maybe Firefox 63 or 64

Preference behind which this will be implemented: 
"layout.css.scrollbar-colors.enabled"

Is this feature enabled by default in sandboxed iframes? Yes.
If not, is there a proposed sandbox flag to enable it? N/A
If allowed, does it preserve the current invariants in terms of what sandboxed 
iframes can do? It's a rendering feature which sandboxed iframes can always be 
simulated with existing rendering features.

DevTools bug: just some color properties, no need specific devtools support?

Do other browser engines implement this? Answer with: mixed signal. We would go 
through the CSS working group and ensure other browser engines are happy with 
them before shipping.

web-platform-tests: There are some web-platform-tests in 
https://github.com/web-platform-tests/wpt/tree/master/css/css-scrollbars
The main rendering of this bug is not testable because scrollbars are very 
platform-dependent, and rendering it like the platform natives is a large point 
of the properties.

Secure contexts: It's not clear. Probably no because it is a rendering feature 
which can always be simulated with existing features.


Also note that, DevTools has been using the properties for dark mode and gave 
positive feedback of them on platforms which they have been available.
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