In gecko, `size` and `content` values aren't implemented yet. So I had to disable these parts with mako. We will need to update the property and the glue once these are implemented in gecko.
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- [X] These changes fix [Bug 1354998](https://bugzilla.mozilla.org/show_bug.cgi?id=1354998)
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Source-Repo: https://github.com/servo/servo
Source-Revision: 7ff803e322a4488eb6b7f813b5f4f82abc49c6c1
Adds parsing / serialization for CSS contain to the style package.
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#15955
- [x] There are tests for these changes
Source-Repo: https://github.com/servo/servo
Source-Revision: 636f8ceb50cef61e34fd5fbd74ed55f098fff72a
