Chrome Version: 69.0.3497.92 (Official Build) (64-bit)
OS: Linux

What steps will reproduce the problem?
(1) Enable "Experimental Web Platform features" flag for VTTRegion support.
(2) Create a TextTrack on a video element.
(3) Create a VTTCue with a VTTRegion applied.
(4) Add CSS styles for ::cue-region

See: https://storage.googleapis.com/shaka-demo-assets/_bugs/firefox-vttregion-style/text.html

What is the expected result?
I should be able to style the region itself (like the background color) with the pseudo-element ::cue-region.

What happens instead?
The style has no effect.