In the WebKit Web Inspector, under the Elements tab, in the right side panel under Styles and then Margin there's a box model UI that is of limited utility to accessibility users. There are multiple ways that this UI could be broken up and annotated, but whatever approach is chosen it would be important that values have relevant information to make it clear what they're associated with. I.e. "position, left, 10". This also has the same issue as https://bugs.webkit.org/show_bug.cgi?id=147459 where a dash is being used to represent no value. This should be replaced before returning accessibilityValue.
<rdar://problem/28228673>