[cssom-view] Fix offsetParent to refer to effective zoom as a stopping point (w3c#10261)

* Update Overview.bs

* Refer to ancestor instead of parent.

* ancestor, not element for containing block.

https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/layout/layout_object.cc;l=4385;drc=6f3f85b321146cfc0f9eb81a74c7c2257821461e;bpv=0;bpt=1

* Fix formatting

* Add period.

Author: chrishtr

cssom-view-1/Overview.bs

@@ -1555,8 +1555,8 @@ The <dfn attribute for=HTMLElement>offsetParent</dfn> attribute must return the
 1. Let <var>ancestor</var> be the parent of the element in the <a>flat tree</a> and repeat these substeps:
     1. If <var>ancestor</var> is <a>closed-shadow-hidden</a> from the element and its computed value of the 'position' property is ''position/fixed'', terminate this algorithm and return null.
     1. If <var>ancestor</var> is not <a>closed-shadow-hidden</a> from the element and satisfies at least one of the following, terminate this algorithm and return <var>ancestor</var>.
-        * The element is a containing block of absolutely-positioned descendants (regardless of whether there are any absolutely-positioned descendants).
-        * The element has a non-default used value of 'zoom'.
+        * <var>ancestor</var> is a containing block of absolutely-positioned descendants (regardless of whether there are any absolutely-positioned descendants).
+        * The element has a different [=effective zoom=] than <var>ancestor</var>.
         * It is <a>the <code>body</code> element</a>.
         * The computed value of the 'position' property of the element is ''static'' and the ancestor is one of the following <a>HTML elements</a>: <code>td</code>, <code>th</code>, or <code>table</code>.
     1. If there is no more parent of <var>ancestor</var> in the <a>flat tree</a>, terminate this algorithm and return null.