[cssom-view] offsetParent should be null for any <body>, not just "the body element" #12834

https://drafts.csswg.org/cssom-view-1/#dom-htmlelement-offsetparent

If any of the following holds true return null and terminate this algorithm:
- [...]
- The element is the body element.

But all browsers agree that the check should be for a <body>. They return null:

• Even if the parent is not a <html>
• Even if they are not the first <body> child of a <html>
• Even if the parent is not the root

https://searchfox.org/firefox-main/rev/bd4c9573f3f4304df50e943f4358a7af7688b4c0/dom/base/Element.cpp#5433
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/layout/layout_object.cc;drc=b0670db20644d78d720e45995089f8d256cb49dd;l=4419
https://searchfox.org/wubkat/rev/0cfebd9e836885d2eb9d3e1faf92ce7e30bc58af/Source/WebCore/rendering/RenderElement.cpp#2270