Auto merge of #29940 - mrobinson:servo-2013-superset, r=Loirooriol

Make Layout 2013 style a superset of Layout 2020 style

This is the first step toward removing the compile-time difference in
style between Layout 2020 and Layout 2013. This change:

- Adds `attr()` CSS content support to legacy layout
- Adds parsing support for some unsupported features such as `display: flow-root` and `display: contents`.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

0 files changed, 0 insertions, 0 deletions