A box with negative margin should drop it in order to fit next to floats rather than push below the floats. This behaviour is not defined by CSS2.1, but the behaviour enforced by this test makes us match IE and FF.

Header
FAIL:
Expected 650 for width, but got 1050. 

<div class="box margin-left" data-expected-width="650">Header</div>
Header
PASS
Header
FAIL:
Expected 650 for width, but got 400. 

<div class="box margin-left" data-expected-width="650">Header</div>
Header
FAIL:
Expected 650 for width, but got 1050. 

<div class="box margin-right" data-expected-width="650">Header</div>
Header
FAIL:
Expected 500 for width, but got 400. 

<div class="box margin-left" data-expected-width="500">Header</div>
Header
PASS
