Mozilla 2002102208 WinXP 1 create a 700px wide table with 3 rows and 1 cell per row 2 insert a 420px wide paragraph in each row 3 - the first paragraph has left and right margins set to 20% - the second paragraph has margins set to auto - the third paragraph has left and right margins set to 140px table, tr, td and p margins and paddings are set to 0 expected result : the three paragraph should be identically centered actual result : the first paragraph is offset 1px to the right, if you increase text size (130% with a default font set to 18px Arial), the box gets correctly centered which makes me think of a rounding problem I will attach a testcase

update to the bug report, this problem seems to happen only when the sidebar is open. With the sidebar closed, it is the reverse problem, the box is correctly centered and the 1px offset problem appears wheb you increase text size.

adding testcase keyword. Still seeing the problem with build 2003042208

This might not be a bug. The problem is that if none of margin, border, padding, content (<p>) and container (<table>) has auto-width, it may be impossible to make this equation work: 'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block (the values are "over-constrained", see CSS 2.1 Specification: 10.3.3) If you insert the values from the first paragraph of the testcase, you get: 20% * tableWidth + 1px + 0px + 420px + 0px + 1px + 20% * tableWidth = tableWidth You can't have all of that (20% margin, 1px border, 0 padding, 420px width) with a 700px table, because 702px != 700px. Mozilla then increases the width of the table element to make room for all that stuff. If you increase the width of the first paragraph's border to, you can see more clearly what happens.

This was fixed since I filed this bug 15 years ago :) None of the Firefox versions we currently ship has this bug.