Positions of overlapping WebVTT cues are resolved in RenderVTTCue. The ultimate coordinate at which the cue is rendered is not expressed as an attribute of the <cue> element.
This would presumably make it easier to test whether cues are being rendered correctly, which would make it possible to catch issues like bug 154240 and bug 136627.
Presumably this is not a WebKit-specific issue. You should file a bug against the WebVTT spec if you think this is a useful change.
Ok, I filed https://github.com/w3c/webvtt/issues/285
<cue> element is not a concept the spec has, so the spec can't set attributes on it. If it's not exposed to author JS or author CSS (e.g. video::cue(cue[top]) should never match) then this would be an implementation detail.