Issue #3230831: Dynamic css generation
We only need this CSS when we're on a node edit form, so the benefits of writing it to disk as a cache are limited.
Rather than getting fancy with event listeners to write to disk at appropriate times, we can drop the CSS into the page as required.
If caching is significantly useful, then I'd probably use Cache API so we're not vulnerable to asset clears removing CSS from a directory where compiled CSS is written to.