[css-tables] Border-collapsing breaking change from 2.1

[gregwhitworth]

This proposal does not respect these stated CSS 2.1 requirements:

UAs must compute an initial left and right border width for the table by examining the first and last cells in the first row of the table. The left border width of the table is half of the first cell's collapsed left border, and the right border width of the table is half of the last cell's collapsed right border. If subsequent rows have larger collapsed left and right borders, then any excess spills into the margin area of the table.

[gregwhitworth]

Adding the additional issue that was in the spec, but is technically related as we're just wanting approval for the changes to how border-collapsing works. But this is doing worse than Edge & Firefox but better than Chrom when row or colspanned cells have border styles which conflict with their neigrs. We feel this is a necessary tradeoff to make the model sane.