[REQ] Allow html attributes besides CSS properties

[mhatzl]

Unimarkup attributes use CSS syntax, and for now only CSS properties are allowed.
This makes it hard to set HTML attributes.

Therefore, HTML attributes should also be allowed in Unimarkup attributes.
Regarding attribute nesting and use in attribute blocks, HTML attributes must be applied to all elements the selector applies to.
However, HTML attributes may not be applied dynamically to elements on a website after elements got changed.

[mhatzl]

It might be possible to allow HTML attributes in media queries, because JavaScript offers matchMedia().

Caveat: This requires JavaScript and is not trivial to implement.

[mhatzl]

Because some HTML attributes like style and href are already covered by the Unimarkup syntax, these should not be allowed.
In general, every element must have a section defining what HTML attributes are allowed.

At some point every element should also have a section defining the allowed CSS attributes.
Since this will take a lot of time, it should not be part of this issue.