2.2.29 [CSS-Level2-2009] Section 10.3.4, Block-level, replaced elements in normal flow
The specification states:
The used value of 'width' is determined as for inline replaced elements. Then the rules for non-replaced block-level elements are applied to determine the margins.
Quirks Mode (All Versions)
This section references [CSS-Level2-2009] Section 10.3.3 for margin calculation, and the equality presented in that section is not supported; therefore, this section’s requirement to match the defined behavior for Section 10.3.3 is also not supported.