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Relies on #17614
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Source-Repo: https://github.com/servo/servo
Source-Revision: a6739cb17f381b88531eb6173d048868aec8e082
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Implements https://html.spec.whatwg.org/multipage/#customelementregistry
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- [X] `./mach build -d` does not report any errors
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- [X] These changes fix#16753 (github issue number if applicable).
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Source-Repo: https://github.com/servo/servo
Source-Revision: b584944f1731c71920abf9a03e1f3cd1e790f7c4
