Much the same, as Nickle suggests. We'd still want something to rest our back, butt and thighs on, so the only difference would be that our shins would be upright in front of us. Quite handy, really, as we could use the soles of our feet as a sort of tray!
With our knees bending the way they do, they just hang off a chair without much effort. So I suppose they'd be like those reclining chairs with a long straight section to rest your lower legs on. After all, if a chair is meant to support your weight then it would be silly to have to hold your legs bent up in front of you, wouldn't it? Incidentally, I reckon that you'd have to swing round into a "backwards" chair rather than just plonk yourself down.