What problem does this solve or what need does it fill?

Wondering if these are useful at all, or even make sense.
They aren't used internally or in any of the examples.

let mut r = UiRect::new(Val::Px(10.), Val::Percent(5.), Val::Auto, Val::VMin(50.));

// Just feels like nonsense; they don't compose or handle failure well
r.left.try_sub_assign(Val::Px(10.).unwrap();
r.right = r.right.try_add(Val::Px(10.)).unwrap_or(r.right.try_add(Val::Percent(10.)));

The try_*_assign_with_size functions seem problematic too:

r.bottom.try_add_assign_with_context(Val::Px(50.), 100., vec2(800., 600.));

r.bottom equals Val::VMin(50.) and is resolved to 600. * 50. / 100. = 300..

So r.bottom is assigned the value: Val::Px(350.).

This might seem like it's okay. But now r.bottom is no longer responsive to changes in the viewport size.