Until someone writes </noscript_within > or < /noscript_within> and your script doesn't catch it but the browser accepts it. Or one of hundreds of other small details that neither one of us has thought of yet.
That's why browser makers have to agree on the acceptable usage and suggest what web devs should search/for and sanitize. Right now, browser makers do not recommend anything except "sanitize your input" and that means something completely different for each browser.