Oh, I didn't realize the `_elem` variant of these parts of the
`Content-Security-Policy` is newer, and so doesn't even work on my
current version of Safari on my Mac.
My rationale at the time was: `script_src_elem` is stricter against
things like imports, and I figured, ok let's do the strictest policy
that works. But since it's not fully compatible with browsers even
*I'm* using right now, and I'm not aware of an actual problem it would
prevent, let's back off that a bit! This should have the same effective
security properties for our case.
Note that the effect of this compatibility issue wasn't *weakening* the
policy; it was being *too* strict, by blocking the scripts and the
stylesheets. This is because `script_src_elem` was ignored, and
`script_src` was absent, so it fell back to `default_src none`.