The serialization of grid shorthands doesn't follow the spec and it makes us fail several WPT tests under css-grid/parsing/. This issue is also present on Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=511177), but it's working fine on Firefox.
Another example: `grid: auto-flow / 200px` currently serializes as `grid: 200px / row` which is invalid and does not parse when round tripping.
<rdar://problem/97952344>