a: th 20% + td 100% — the column takes the MAX of its cells.
Chrome: table=600, cells 479,66,55. This is investing.com's quote table.

NameLastChg.
Dow Jones54,036.93+151.83

b: th 20% only — Chrome: table=362, cells 72,157,132

NameLastChg.
Dow Jones54,036.93+151.83

c: td 100% only — Chrome: table=600, cells 479,66,55

NameLastChg.
Dow Jones54,036.93+151.83

d: fixed px vs a later 100% — Chrome takes the percentage (600).
NetSurf keeps its long-standing "a fixed width outranks a percentage" rule
and gives 241 here. Known difference, left alone: no real page does this.

NameLastChg.
Dow Jones54,036.93+151.83

e: two fixed widths in one column — the larger wins.
Chrome: table=274, cells 208,66

NameLast
Dow Jones54,036.93