Per https://html.spec.whatwg.org/multipage/webappapis.html#creating-a-classic-script , the input source is supposed to be a string, which, per https://infra.spec.whatwg.org/#string is

A string is a sequence of unsigned 16-bit integers, also known as code units. A string is also known as a JavaScript string. Strings are denoted by double quotes and monospace font.

This is distinct from a "byte string", i.e. AK::StringView/AK::DeprecatedString, and from a UTF-8 string aka AK::String. The spec suggests the input should be Span<u16>. Which seems quite gross.

If we intend to implement the algorithm taking a UTF-8 string, we should make that explicit with implementation note comments. And if we intend to make assumptions about the UTF-8-ness of a string in that algorithm, we should validate the input being UTF-8 either before calling it, after calling it, or as part of another spec step, such as the one that calls https://tc39.es/ecma262/#sec-parse-script (see #17899)