[css-transforms-2] Make backface-visibility:hidden create a stacking context and containing block when participating in a 3D rendering context. (#918) (#9348)

Co-authored-by: Matt Woodrow <m_woodrow@apple.com>

Author: mattwoodrow

css-transforms-2/Overview.bs

@@ -933,6 +933,11 @@ Computed value: specified keyword
 Animation type: discrete
 </pre>
 
+A computed value of ''backface-visibility/hidden'' for 'backface-visibility'
+on a <a>transformable element</a> that participates in a <a>3D rendering context</a>
+establishes both a stacking context and
+a [=containing block for all descendants=].
+
 The visibility of an element with ''backface-visibility: hidden'' is determined as follows:
 
 1. Compute the element's <a>accumulated 3D transformation matrix</a>.