Content is not well-formed on 404 page

🐛 css_defer_external option does not work if CSS URL contains characters not allowed by the regex Needs review started converting the logic to use DOM instead of a regexp, so we should probably focus on getting that committed and then we can check if the current issue still applies.

We used the patch from #4 but the issue was back on D10.1.
I adjusted the regex with the changes i found in https://www.drupal.org/project/advagg/issues/3377084 🐛 css_defer_external option does not work if CSS URL contains characters not allowed by the regex Needs review and now the 403/404 page shows correct again.
Without the changes i get whitescreen since the "old" regex leads to a "Backtrack Limit Exhausted" error which leads en empty content variable afterwards which results in the whitescreen.