> (X)HTML should have support for multiclassing, which

It already does, although it is broken in some earlier
browsers, and some common authoring tools also generate multiple
classes as though they were one class with a name containing
spaces.

> can deflate code size. It is especially useful when
> dynamically changing style through the DOM (i.e. with
> JavaScript.) A sample markup snippet would be:

If you are really complaining about the W3C DOM not allowing easy 
manipulation of this construct, you are on the wrong mailing
list.  Also note that classes are not specific to CSS (and CSS has
its own list); a document can enhance its semantics with classes without
any author provided style sheet or DOM manipulation.

> <li classes="nav-item,active">Section 2</li>

This should be (assuming "-" is allowed, which I haven't checked):

] <li class="nav-item active">Section 2</li>