This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
"The 'padding-top' and 'padding-bottom' property on the WebVTT cue background box must be set to '1.5vh' or another user-agent-defined number to define a margin at the edges of the video into which cues will not be placed." One way of fixing this is to not use padding at all, instead we could use the (future) machinery for regions and allow the browser to use a smaller implicit region.
We might need both - to get padding around the text in the cues and padding for the video safe region. But you're right - it should be called "padding" and not "margin".
Wording fixed in https://github.com/w3c/webvtt/pull/32