Drop "value" from "WebVTT percentage value"

The only other bits of syntax to include "value" are those with a
companion "name", but such is not the case here.
This paragraph is not necessary.

The check is redundant, because both the percentage string parsing and
the "Otherwise" branch will deal with such a condition.
Merge pull request #21 from silviapfeiffer/remove-unnecessary

This paragraph is not necessary.
Merge pull request #22 from foolip/percentage-value

Drop "value" from "WebVTT percentage value"
Remove 'text-outline' from CSS whitelists

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22149
Merge pull request #23 from foolip/text-outline

Remove 'text-outline' from CSS whitelists
Round the line position to an integer for snap-to-lines cues

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24420
Thank Caitlin Potter
Merge pull request #24 from foolip/thank-caitlin

Thank Caitlin Potter
Make VTTRegion and VTTRegionList not inherit EventTarget

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22962

Thanks Kyle!
Merge pull request #25 from foolip/round-line-position

Round the line position to an integer for snap-to-lines cues
Merge pull request #26 from foolip/no-eventtarget

Make VTTRegion and VTTRegionList not inherit EventTarget
Thank Rick Eyre
Make VTTRegion.scroll an enum ScrollSetting
Remove dead code throwing SyntaxError in enum setters

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22967
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22968
Avoid overlap in a regions example

https://www.w3.org/Bugs/Public/show_bug.cgi?id=23267
Make TextTrack.regions readonly

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22963
Merge pull request #30 from foolip/readonly-regions

Make TextTrack.regions readonly
Merge pull request #27 from foolip/thank-rick

Thank Rick Eyre
Merge pull request #28 from foolip/enum-syntax-error

Make VTTRegion.scroll an enum ScrollSetting

Remove dead code throwing SyntaxError in enum setters
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22967
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22968