[cssom-view-1] Per WG resolution, find the bounding box of all fragments. w3c#6588

Author: tabatkins

cssom-view-1/Overview.bs

@@ -1399,13 +1399,17 @@ The <dfn attribute for=HTMLElement>offsetLeft</dfn> attribute must return the re
 The <dfn attribute for=HTMLElement>offsetWidth</dfn> attribute must return the result of running these steps:
 
 1. If the element does not have any associated <a>CSS layout box</a> return zero and terminate this algorithm.
-1. Return the <a>border edge</a> width of the element's [=principal box=],
+1. Return the width of the axis-aligned bounding box
+    of the [=border boxes=]
+    of all fragments generated by the element's [=principal box=],
     ignoring any <a>transforms</a> that apply to the element and its ancestors.
 
 The <dfn attribute for=HTMLElement>offsetHeight</dfn> attribute must return the result of running these steps:
 
 1. If the element does not have any associated <a>CSS layout box</a> return zero and terminate this algorithm.
-1. Return the <a>border edge</a> height of the element's [=principal box=],
+1. Return the height of the axis-aligned bounding box
+    of the [=border boxes=]
+    of all fragments generated by the element's [=principal box=],
     ignoring any <a>transforms</a> that apply to the element and its ancestors.