Clarify what happens when not rendered

Author: chrishtr

cssom-view-1/Overview.bs

@@ -1439,7 +1439,7 @@ The <dfn attribute for=Element>clientHeight</dfn> attribute must run these steps
 1. Return the [=unscaled=] height of the <a>padding edge</a> excluding the height of any rendered scrollbar between the <a>padding edge</a> and the <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
 
 The <dfn attribute for=Element>currentCSSZoom</dfn> attribute must return the
-[=effective zoom=] of the element.
+[=effective zoom=] of the element, or 1.0 if the element isn't [=rendered=].
 
 <h3 id=element-scrolling-members>{{Element}} Scrolling Members</h3>