The Github page doesn't give any examples that look like OO to me, which is A) not surprising, because what the hell would an OO markup language look like and B) very surprising, given that the whole DOM is OO from the ground up in modern rendering engines.
Anyway, on a tangent...
I see no real call for OO in web rendering, but the one thing I think is missing from HTML is the ability to parameterise things like column widths etc. Why cant I call column 1's width "x" and ask the renderer to make column 2's width "3x"? Or use these parameters across tables, so that the columns in table 1, table 2 and table 3 are all the same size?
I know this can be done with CSS, but in order to do that, I need to choose a particular size -- I can say "I don't care about the actual size, but these 3 things should all be as big as each other."