with tabs:

test abcdefg hijklm nopqr stuv wxy z0 1

cell 2

with spaces:

test abcdefg hijklm nopqr stuv wxy z0 1

cell 2

In Firefox 3.0 (Windows), tabs appear not to contribute anything to the width -- the cell is just wide enough for all the non-tab characters:

In Firefox 2.0.0.14 (Windows), I see this: