My previous commit introduced this. Handling `\` outside strings before
readIdentifier meant a LEADING escape was eaten before the url check
saw it: `\75` is the CSS escape for `u`, so `.a{background:\75rl(...)}`
is url() to a browser and passed through untouched, with no warning —
a bypass the base version did not have. An escape mid-identifier
(`u\72l`) was unaffected, which is why the first tests missed it.
The escape branch now runs AFTER readIdentifier, which already decodes
leading escapes itself. What is left for it is the case it was added
for: `\'`, which must not be read as opening a string.
Both spellings are pinned, along with the legitimate escaped selector
and data: URI that must survive untouched.
Refs #1264
Paul Nothaft
b6dc0991ce