* Implement support for `drop-shadow`
* Clean up remnant from early attempts
* Fix misleading comments on GenericSimpleShadow
If Servo-specific `style` changes will need to be upstreamed anyway, I might as well fix a thing that had thrown me off!
* Revert "Fix misleading comments on GenericSimpleShadow"
This reverts commit cdc810b826ac082041adc212c24649ee3b86ca0a.
* Clean up an import
* Update test expectations
* Fix missing expectation on Layout 2013
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.