4.6 [CSS-Level1-2008] Section 5.5.23, width

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V0907:

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.