Reacting to environment changes
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---
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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 a subset of #11416
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- [x] These changes do not require tests because they will be added when the complete responsive images is complete
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Implement document load cancellation
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---
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix#19309fix#21114fix#21113 (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/21111)
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Implement update source set & select image source
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---
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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#11416 (github issue number if applicable).
- [x] These changes require tests but cannot be written until implementation of responsive images is complete
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/21181)
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Implement beforeunload event and infrastructure
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Implementation of:
1. https://html.spec.whatwg.org/multipage/#prompt-to-unload-a-document, and
2. https://html.spec.whatwg.org/multipage/#unload-a-document
---
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix#10787 and fix#20485 and fix#20588 and fix#20496 (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____
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