2.1.46 [CSS-Level2-2009] Section 10.3.3, Block-level, non-replaced elements in normal flow
If all of the above have a computed value other than 'auto', the values are said to be "over-constrained" and one of the used values will have to be different from its computed value. If the 'direction' property of the containing block has the value 'ltr', the specified value of 'margin-right' is ignored and the value is calculated so as to make the equality true. If the value of 'direction' is 'rtl', this happens to 'margin-left' instead.
Quirks Mode (All Versions)
When one or more of the listed properties are
"over-constrained" and the value of the direction property is
ltr, the margin-right property is not adjusted.
Instead, both margins and paddings are applied and the content box is reduced.
IE8 Mode (All Versions)
The value of the margin-left property is not
recalculated and the value for margin-right is not set to 0 when the direction
property is set to