With Firefox 57 on Mac OS X
With Firefox preferences for colors set to display high contrast (black text, white background)
With Firefox loading this page: https://build.fluidproject.org/videoPlayer/demos/Mammals.html
The current behaviour is the interface components on the video player are rendered invisible. I tried with white text and black background).
The expected behaviour is that that video controller interface elements should still be visible even when the user has high contrast color settings
Not sure how you would prioritize this.