The body has 120px horizontal margins, the green box is right floated, the red one left floated.
1a. The blue box has overflow:hidden
2a. The blue box has overflow:hidden and a positive left margin.
3a. The blue box has overflow:hidden and a negative left margin.
4a. Like 3a, without the float
5a. Like 3a, without overflow:hidden
6a. Like 3a, with a width on the overflow box.
7a. The blue box has display: table; and a negative left margin.
1b. The blue box has overflow:hidden
2b. The blue box has overflow:hidden and a positive right margin.
3b. The blue box has overflow:hidden and a negative right margin.
4b. Like 3b, without the float
5b. Like 3b, without overflow:hidden
6b. Like 3b, with a width on the overflow box.
7b. The blue box has display: table; and a negative right margin.
Safari 4 is correct.
Opera 10.50 is almost correct, except in case 3b. Previous versions of Opera had more problems.
Firefox 3 (not 2) and IE8 (not 7) have a problem in cases 3a and 3b: they ignore the negative margin. In Firefox 3 this happens in 6a, 6b, 7a, 7b as well.