`wp_html_split` valid HTML attributes issues

[soulseekah]

There are a handful of valid HTML attributes that shatter wp_html_split.

Since it works by looking for the < character we can break it in many ways, starting from:

​https://mathiasbynens.be/demo/crazy-class
​https://mathiasbynens.be/demo/html5-id

And ending in the less exotic and crazy:

<span data-content="<p>abcd</p>">loading...</span>

Same goes for CSS attribute selectors in <style> tags.

Related #43456, #39153, #40191