Rounder corners, fixed card grids, numbered lists, decorated underlines

A round of base-layer rendering fidelity, each fix checked pixel for pixel against the reference browser. Corners are now fully modelled: a border-radius can differ per corner and be elliptical, so a shape with 50 percent rounding on a wide box is a true ellipse rather than a stadium pill, and asymmetric corners like a chat bubble render correctly. A common and glaring layout bug is gone: a responsive grid of padded cards used to overflow its cells and merge into one solid block, and now each card sits in its own cell with the right gaps. Custom numbered lists work again, with counters that render in decimal, roman or alphabetic styles for outlines and step-by-step guides. And underlines can now take a style, so the wavy, dotted, dashed and double underlines that modern links and emphasis use render properly instead of as a plain straight line.