2.1.53 [CSS-Level2-2009] Section 9.9.1, Specifying the stack level: the 'z-index' property

V0080:

The specification states:

 z-index
  
 Value:                 auto | <integer> | inherit
 Initial:               auto
 Applies to:            positioned elements
 Inherited:             no
 Percentages:           N/A
 Media:                 visual
 Computed value:        as specified 
  
 For a positioned box, the 'z-index' property specifies:
§   The stack level of the box in the current stacking context. 
§   Whether the box establishes a stacking context.

Quirks Mode and IE7 Mode (All Versions)

The initial value of the z-index property is 0.

IE8 Mode (All Versions)

The z-index property resolves large negative values (such as -2147483646) to 'auto'.

The z-index property accepts number values and percentage values instead of only allowing integers.

V0081:

The specification states:

 Auto
 The stack level of the generated box in the current stacking context is '0'. The
  box does not establish a new stacking context unless it is the root element.

Quirks Mode and IE7 Mode (All Versions)

The auto value is mapped to '0' so that the initial value of the z-index property is '0'.

V0082:

The specification states:

 The root element forms the root stacking context. Other stacking contexts are 
 generated by any positioned element (including relatively positioned elements) 
 having a computed value of 'z-index' other than 'auto'. Stacking contexts are not 
 necessarily related to containing blocks.

Quirks Mode (All Versions)

Assigning an element a relative, absolute, or fixed position generates a stacking context.

IE7 Mode (All Versions)

Assigning an element a relative or absolute position generates a stacking context.

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