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Add methods to activatable elements; gated by a preference, for use in servo-specific WPT tests.
I cargo-culted over the "redundant check" from #12322, I don't really understand the possible exploit.
Obvious flaw is the copypasta for each element. I understand that the webidl codegen will produce a trait for ActivatableElement, but I can't see how I can provide a default implementation for this? I guess otherwise could be an improvement to have each element just delegate to an Enter.../Exit... method on the Activatable trait?
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#12343 (github issue number if applicable).
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- [X] There are tests for these changes OR
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…states
Source-Repo: https://github.com/servo/servo
Source-Revision: 2fb4dd9ec3bb4be7d5ea8404964536c89fc8732b
