It looks like we can trivially fix this by just changing the `lines` attribute in our webIDL to an `unsigned long` instead of a `long`: https://searchfox.org/mozilla-central/source/dom/webidl/VTTRegion.webidl#24
Bug 1565737 Comment 1 Edit History
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It looks like we can trivially fix this by just changing the `lines` attribute in our webIDL to an `unsigned long` instead of a `long`: https://searchfox.org/mozilla-central/source/dom/webidl/VTTRegion.webidl#24 Edit: no, I was wrong. The WPT is expecting something that doesn't make sense for an attribute that is meant to be an `unsigned long`, and I can't figure out what the correct fix is supposed to be based on the GitHub issue :alwu mentioned in comment #0.