The new spec that pass a11y review is the following:
box-shadow: 0px 0px 1px rgba(128, 128, 142, 0.9), 0px 0px 4px rgba(128, 128, 142, 0.5);
Please be sure that the color for the selected tab is white (#fff) and the background color for the tab strip is #F0F0F4.
What we're trying to ensure here is that the selected tab, which has a white background, is distinguishable from the toolbar background. The border/shadow enhances the contrast between the selected white tab and the light gray toolbar background by darkening the toolbar background immediately surrounding the selected tab. The critical contrast ratio is between the selected tab, which is pure white, and the toolbar color which is now #B4B4BD at its darkest point. That gives us a contrast ratio of about 2:1 which is significantly better than the other browsers provide. But there's a "but" which is that our dark border/shadow is quite thin where it's the darkest -- one pixel, and that one darkest surrounding pixel may not be perceivable by some with worse vision. That means we should really be looking at the sort of average of several pixels worth of border/shadow/background to get a better sense of the contrast users with less acute vision will experience. When I take the average of several pixels that surround the selected tab, I get something like #BCBCC5 which gives the selected button a contrast ratio of about 1.9:1 against the toolbar. That's still better than the other browsers and definitely an improvement on the prior design. I think that's good.
Dark theme spec will arrive later today or tomorrow EU time.