This is too wide. It'll also block things like `*:before { content: "url container"; }`. Instead the trimmed value should start with either url(...) or a comma, then zero or more whitespace characters, then url(...), so we end up with a regexp like this:
```
if (value.trim().match(/,/\s*url([^)]+)/))
```
but you'll have to double check I didn't typo anything there. Easy to do that with regexp =x