Rephrase misleading recommendation to escape "<script" as "<\script" in literals

[mathiasbynens]

https://html.spec.whatwg.org/multipage/scripting.html#restrictions-for-contents-of-script-elements

The easiest and safest way to avoid the rather strange restrictions described in this section is to always escape an ASCII case-insensitive match for "<!--" as "<\!--", "<script" as "<\script", and "</script" as "<\/script" when these sequences appear in literals in scripts (e.g. in strings, regular expressions, or comments) […]

It’s misleading to state that this is the way to do it in all literals including regular expression literals, since \s (in <\script) has a different meaning in regular expression literals (it matches whitespace instead of the character s). We could either suggest a different way of escaping this (e.g. "<script" as "\x3Cscript") or clarify that for regular expression literals specifically a different approach should be used.

[koto]

Perhaps a different letter from script could be picked up that doesn't class with RE's metacharacters?

[mathiasbynens]

Perhaps a different letter from script could be picked up that doesn't class with RE's metacharacters?

I think this is also undesirable since that is disallowed in regular expressions with the u flag. E.g. /\i/u throws. Since we’d need to use an escape sequence of the form \xNN or \uXXXX anyhow, any character seems fine, so it might as well be < → \x3C (to still keep the rest of the pattern somewhat readable).

[zcorpan]

Escaping < instead as you suggested SGTM. We can recommend that for all of the strings here for consistency.