valueStart will need a correction, because multiple `url('...')` means that `valueStart`, which points to the first `url('...')`may not be where the error really is. Test case:
```
<style>
*{
  background: url('https://...'),
              url('http://...')
}
</style>
```

(use `var checkMixedContent = true || (window.location.protocol === "https:");` at the top of slowparse for localhost testing, then add that to a new thimble to see the misplaced cursor atm)