4.6 [CSS-Level1-2008] Section 5.5.23, width
The [CSS-Level1-2008] specification states:
Width Value: <length> | <percentage> | auto Initial: auto Applies to: block-level and replaced elements Inherited: no Percentage values: refer to parent element's width
All Document Modes (All Versions)
With the exception of absolutely positioned elements in quirks mode, percentage widths are calculated based on the width of the containing block.
IE7 Mode, IE8 Mode, and IE9 Mode (All Versions)
For absolutely positioned elements whose containing block is based on a block-level element, the percentage width is calculated with respect to the width of the padding box of the element.