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Added code for initial steps in html form validation.
1. Added methods for trait validatable
2. implemented stub methods for elements like HTMLInputElement, HTMLButtonElement, etc
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- [X ] `./mach build -d` does not report any errors
- [ X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (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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Source-Repo: https://github.com/servo/servo
Source-Revision: beec035eb0ff6babf48ce836184329e60e795177