[css-transforms-2] Describe 3D sorting more precisely.

This describes the handling of descendants in 3D sorting more clearly,
and also doesn't limit its reference to Appendix E to steps 1-7 (which
excludes steps 8-10, which might be non-empty).

Removes an internal issue in the document, and fixes #926.

Author: dbaron

css-transforms-2/Overview.bs

@@ -321,16 +321,24 @@ Elements establish and participate in 3D rendering contexts as follows:
 Some CSS properties have values that are considered to force "grouping": they require that their element and its descendants are rendered as a group before being composited with other elements; these include opacity, filters and properties that affect clipping. The relevant property values are listed under <a href="#grouping-property-values">grouping property values</a>. Consequently, when used on an element with transform-style:preserve-3d, they change the used value to
 ''transform-style/flat'' and prevent it from creating or extending a <a>3D rendering context</a>.
 
-Issue: This doesn't describe how untransformed content within a 3D transformed element is handled, the algorithm probably needs to be recursive.
-Issue: Figure out what the current behavior of various engines is, and figure out a clearer description that wont break web compat.
-
-The rendering of elements in a 3D rendering context is as follows (numbers refer to items in <a href="https://www.w3.org/TR/CSS2/zindex.html#painting-order">CSS 2.1, Appendix E, Section E.2 Painting Order</a>):
-
-<ol style="list-style-type: upper-alpha;">
-	<li>The background, borders, other box decorations, and content of the establishing element, and its descendant elements without 3D transforms, ordered according to steps 1—7, are rendered into a plane at z=0 relative to the establishing element.
-	<li>3D-transformed elements are each rendered into their own plane, transformed by the <a href="#accumulated-3d-transformation-matrix-computation">accumulated 3D transformation matrix</a>.
-	<li>Intersection is performed between the set of planes generated by steps A and B, according to <a href="http://en.wikipedia.org/wiki/Newell%27s_algorithm">Newell's algorithm</a>.  Coplanar [=3D transformed elements=] are rendered in painting order.
-</ol>
+In a 3D rendering context, rendering and sorting of elements is done as follows:
+
+1. The element establishing the 3D rendering context,
+	and each other 3D transformed element participating in the 3D rendering context,
+	is rendered into its own plane.
+	This plane includes
+	the element's backgrounds, borders, other box decorations, content,
+	and descendant elements,
+	excluding any descendant elements that have their own plane (and their descendants).
+	This rendering is done according to
+	<a href="https://www.w3.org/TR/CSS2/zindex.html#painting-order">CSS 2.1,
+	Appendix E, Section E.2 Painting Order</a>.
+2. Intersection is performed between this set of planes,
+	according to
+	<a href="http://en.wikipedia.org/wiki/Newell%27s_algorithm">Newell's algorithm</a>,
+	with the planes other than that of the establishing element
+	transformed by the [=accumulated 3D transformation matrix=].
+	Coplanar [=3D transformed elements=] are rendered in painting order.
 
 Issue: is it OK to not pop 2D-transformed elements into their own planes?
 
@@ -1443,5 +1451,6 @@ Recent Changes</h3>
 		<li>Define that clamping of values of ''perspective()'' also applies to resolved values and interpolation (see <a href="https://github.com/w3c/csswg-drafts/issues/6320">#6320</a> and <a href="https://github.com/w3c/csswg-drafts/issues/6346">#6346</a>).</li>
 		<li>Clarify that the effects of ''transform-style/preserve-3d'' only affect <a>transformable elements</a> (see <a href="https://github.com/w3c/csswg-drafts/issues/6430">#6430</a>).</li>
 		<li>Added a note that the resolved value of 'translate' includes percentages (see <a href="https://github.com/w3c/csswg-drafts/issues/2124">#2124</a>).</li>
+		<li>Describe 3D sorting more precisely, to explain which descendants are included, and not limit Appendix E reference to steps 1-7 (see <a href="https://github.com/w3c/csswg-drafts/issues/926">#926</a>).</li>
 	</ul>