Parse inline style attributes into the layout-relevant subset of
LayoutStyle: display, width/height, margin, padding, and border width,
including box shorthands (1-4 values) and per-side longhands. !important
is stripped; auto and percentages leave the size unset for phase 1.
Layered on a small built-in UA default sheet. A compact hand-rolled
tokenizer handles inline attributes without a cssparser dependency; a
real cascade can harden it later.