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"Cues whose text track cue snap-to-lines flag is set will be placed within the title-safe area on user agents that use overscan. Cues with the flag unset will be positioned as requested (modulo overlap avoidance if multiple cues are in the same place)." Authors need a way to avoid overscan problems when using non-snap-to-line cues. WebVTT should interpret percentage values as relative to the safe region.
We're planning to deal with title-safe areas/overscan in a different way, see bug 22085 . Would value your thoughts.