This effectively makes it per-Document, but we hang it off of
StyleComputer since that's what it's used for.
The purpose of this is to prevent downloaded fonts from escaping the
context that loaded them. There's probably a more elegant solution where
we still share caching of system fonts, but let's start here.
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 *
