SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
Since we already have a type enum for these, let's use it to make
is<T> bypass dynamic_cast for CSS rules.
These were often near the top of random browser profiles.
