2.1.65 [CSS-Level2-2009] Section 10.6.5, Absolutely positioned, replaced elements
This situation is similar to the previous one, except that the element has an intrinsic height. The sequence of substitutions is now: 1. The used value of 'height' is determined as for inline replaced elements. If 'margin-top' or 'margin-bottom' is specified as 'auto' its used value is determined by the rules below. 2. If both 'top' and 'bottom' have the value 'auto', replace 'top' with the element's static position. 3. If 'bottom' is 'auto', replace any 'auto' on 'margin-top' or 'margin-bottom' with '0'. 4. If at this point both 'margin-top' and 'margin-bottom' are still 'auto', solve the equation under the extra constraint that the two margins must get equal values. 5. If at this point there is only one 'auto' left, solve the equation for that value. 6. If at this point the values are over-constrained, ignore the value for 'bottom' and solve for that value.
Quirks Mode and IE7 Mode (All Versions)
An absolutely-positioned replaced element with a percentage height that cannot be resolved is collapsed to a height of 0.
Quirks Mode (All Versions)
The following clarifications apply:
The height of the replaced element is not restricted to 150px.
A percentage height that cannot be resolved using these rules is instead calculated with respect to the initial containing block.