Processing non-ascii tags and attributes in HTMLParser

[serhiy-storchaka]

Bug report

html.parser.HTMLParser convert names of tags and attributes to lower case. But the HTML5 specification only prescripts converting ASCII upper alpha characters to lower case.

• https://html.spec.whatwg.org/multipage/parsing.html#tag-name-state
• https://html.spec.whatwg.org/multipage/parsing.html#attribute-name-state

There are some non-ASCII characters which are converted to ASCII lowercase characters (e.g. "ß" -> "ss", "K" (U+212A) -> "k", "ſ" -> "s"). They will be parsed differently by HTMLParser and any other parser or browser.

Linked PRs

• gh-141756: Fix handling non-ASCII element and attribute names in HTMLParser #141758