2.2.30 [CSS-Level2-2009] Section 10.3.6, Floating, replaced elements
The specification states:
If 'margin-left' or 'margin-right' are computed as 'auto', their used value is '0'.
The used value of 'width' is determined as for inline replaced elements.
Quirks Mode (All Versions)
Floating replaced elements are offset by 3px and not by 0px.