"
The offsetParent attribute must return the result of running these steps:
1.If any of the following holds true return null and terminate this algorithm:
(...)
The element's computed value of the 'position' property is 'fixed'.
"
now written at
8 Extensions to the HTMLElement Interface
http://www.w3.org/TR/cssom-view/#extensions-to-the-htmlelement-interface
Bug 434678 Comment 5 Edit History
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"
If any of the following holds true return null and terminate this algorithm:
(...)
The element’s computed value of the position property is fixed.
"
https://www.w3.org/TR/cssom-view/#dom-htmlelement-offsetparent
now written at
7. Extensions to the HTMLElement Interface
https://www.w3.org/TR/cssom-view/#extensions-to-the-htmlelement-interface