Like nsCSSParser and mozilla::css::ErrorReporter, we need to send CSS parsing errors to a page's developer tools console or the browser console.
These mochitest failures are for this: https://dxr.mozilla.org/mozilla-central/rev/00a166a8640dffa2e0f48650f966d75ca3c1836e/layout/style/test/stylo-failures.md#82-84
I'm giving this a shot. I'll unassign myself if I'm not making progress.
Awesome, thanks jdm!
Created attachment 8859633 [details] [diff] [review] WIP I made a bunch of progress during a 14 hour plane ride! CSS parsing errors are now appearing in the console for operations like "element.style = 'foopy'"; I'm going to look into the automated tests next, as well as sort out how to get the stylesheet URL from stylo into the error reporter.