Empty row-groups special treatment

Empty row-groups are treated differently during layout. Spec does not mention this. The issues I've found are:

1. When is row-group treated as empty?

Only row-groups with no children are treated as empty. Empty row child, or an abspos child, all trigger non-empty behavior.

2. What is the size of an empty row-group?

external/wpt/css/css-tables/bounding-box-computation-3.html asserts that empty row-group should have width of 0

Chrome 87, FF 83, Safari 13 disagree, the row-group width matches other rows.
Pre-Chrome Edge has width of 0.

Suggestion: standardize empty row-group width to match table's grid, like other rows. This is prevalent implementation, and aligns with CSS behavior for empty blocks.

3. border-spacing and empty row-groups?

    <table border-spacing:10px>
      <tbody></tbody>
      <tbody></tbody>
      <tbody><td>x</td></tbody>
    </table>

Chrome 87, Edge 17, Safari 13 do not generate border spacing for empty row groups. FF does.

What is the location of the first tbody? I believe its offsetTop should be 0 (no border spacing), but bounding-box-computation-3.html disagrees. It inserts border spacing before first row-group, even if it is empty. Unless all row-groups are empty, in which case there is no border-spacing.
Suggestion: empty row-groups should not generate any border spacing before or after ever.

Start with external/wpt/css/css-tables/bounding-box-computation-3.html and review its assertions for empty row-groups, widths, and border-spacing. Compare those expectations with the issue's reported browser behavior. Done means the CSS Tables specification clearly defines these cases and the relevant test reflects the agreed behavior.