I see the following HTML code used a lot to load jQuery from a content delivery network, but fall back to a local copy if the CDN is unavailable (e.g. in the Modernizr docs):
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.js"></script>
<script>window.jQuery || document.write('<script src="js/libs/jquery-1.6.1.min.js">\x3C/script>')</script>
My question is, why is the last < character in the document.write() statement replaced with the escape sequence \x3C? < is a safe character in JavaScript and is even used earlier in the same string, so why escape it there? Is it just to prevent bad browser implementations from thinking the </script> inside the string is the real script end tag? If so are there really any browsers out there that would fail on this?
As a follow-on question, I've also seen a variant using unescape() (as given in this answer) in the wild a couple of times too. Is there a reason why that version always seems to substitute all the < and > characters?