I like it. Nice effort. Plus I like the visual style a lot too.

I feel there's a mismatch between creating novel "semantic" elements, and then customising them in the markup, rather than the contextual approach (nesting, rich selectors). The mismatch is that the new elements still apply a "what" approach, but the attributes used for customisation apply a "how" approach and leave it in the mark-up. It's still like `<p class="red" />` rather than `main p { background-color: red; }`.

I get that there's a trade-off between purity and code that's nice to work with, and I think you've hit a very readable, appealing and creative balance.