ParseBorderProperty fails on default included CSS

Specifically, it takes hr { border: 1px inset; } and passed 1px into ParseBorderColorProperty

Start by locating ParseBorderProperty and ParseBorderColorProperty, then reproduce the issue with the included CSS rule `hr { border: 1px inset; }`. Trace why `1px` is sent to the color parser; done means the shorthand is parsed without treating the width as a color, with the relevant parsing behavior covered by a regression test.

索引モデルが issue の本文から書いたものです。