If I understand the HTML spec correctly, these two examples are not valid HTML. The script element content model has the constraint that if the substring "<!--

If I understand the HTML spec correctly, these two examples are not valid HTML. The script element content model has the constraint that if the substring "<!--" occurs inside the content, a corresponding "-->" must occur before the </script> end-tag. (Technically, this would not be a comment though, since it is not discarded but parsed as character data.) If I put the first example through the (X)HTML validator (https://html5.validator.nu/ https://html5.validator.nu/) I get: > Error: The text content of element script was not in the required format: Content contains the character sequence <!-- without a later occurrence of the character sequence -->. I suspect this constraint on the script-element is exactly to avoid these parsing/lexing ambiguities. A regex would still have to special-case script elements (and also style-elements I guess?) because content can contain unescaped "<". But this can still be done using a regex, as far as I can tell, since there is still no recursive productions in the syntax. I don't believe there is any two-way interaction between the HTML parser and the JavaScript parser. The HTML parser passes the character data content of the script element to the JavaScript parser, but the JavaScript parse does not have any effect back on the HTML parse. (After all, it is legal for a user agent to not support JavaScript, but this should obviously not affect the parsing of the HTML.) (Thanks for sending me down this rabbit-hole. HTML is weird!)