testdesc">Assignable: 400px, C0:40% C1:20% C2:40%
Percentage header cells with colspan.
Colspan cells do not distribute border_padding, they just distribute widths.
C0/1/2/3 will not have border-padding becaouse of colspan, C4 because border-box
TD0 splits
C0/1 get 20% max (40 + padding)/ 2 = 22px,
C2/3 10% and (20 + padding) / 2 = 12px max,
C4 40%, 12px percent border padding, and 52 max

Assignable width is 440 - 48, everyone gets according to percent.
C0/C1 get 80, C2/C3 get 40, and C4 gets 160.
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