[css-overflow][css-contain] define and use overflow clip edge (w3c#5134)

Follow-up / rephrasing of w3c#4934

Author: frivoal

css-contain-1/Overview.bs

@@ -733,6 +733,12 @@ Paint Containment</h3>
 		nor does it inhibit the creation of any such mechanism through other properties,
 		<span class=informative>such as 'overflow', 'resize', or 'text-overflow'</span>.
 
+		Note: The next level of this specification [[CSS-CONTAIN-2]] refines this effect
+		to apply to the [=overflow clip edge=] rather than the [=padding edge=],
+		in order to take the new 'overflow-clip-margin' property into account.
+		For implementations that do not support 'overflow-clip-margin',
+		the effect is identical.
+
 		Note: The behavior is described in this paragraph is equivalent to changing
 		''overflow-x: visible'' into ''overflow-x: clip''
 		and

css-contain-2/Overview.bs

@@ -873,14 +873,14 @@ Paint Containment</h3>
 
 	1. The contents of the element
 		including any [=ink overflow|ink=] or [=scrollable overflow=]
-		must be clipped to the <a>padding edge</a> of the [=paint containment/containing box=],
+		must be clipped to the <a>overflow clip edge</a> of the [=paint containment/containing box=],
 		taking <a>corner clipping</a> into account.
 		This does not include the creation of any mechanism
 		to access or indicate the presence of the clipped content;
 		nor does it inhibit the creation of any such mechanism through other properties,
 		<span class=informative>such as 'overflow', 'resize', or 'text-overflow'</span>.
 
-		This clipping shape respects 'overflow-clip-margin',
+		Note: This clipping shape respects 'overflow-clip-margin',
 		allowing an element with [=paint containment=]
 		to still slightly overflow its normal bounds.
 

css-overflow-3/Overview.bs

@@ -383,9 +383,8 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 
 		<dt><dfn>clip</dfn>
 		<dd>
-			Like ''hidden'',
-			this value indicates that
-			the box’s content is clipped to its padding box
+			This value indicates that
+			the box’s content is clipped to its [=overflow clip edge=]
 			and that no scrolling user interface should be provided by the UA
 			to view the content outside the clipping region.
 			In addition, unlike ''overflow: hidden''
@@ -511,19 +510,20 @@ Expanding ''overflow:clip'' Bounds: the 'overflow-clip-margin' property</h3>
 	the element is allowed to paint
 	before being clipped.
 
-	Values are defined as follows:
+	The <dfn export>overflow clip edge</dfn> initially coincides with the [=padding edge=],
+	but can be expanded outwards by 'overflow-clip-margin',
+	as defined below.
 
+	Values are defined as follows:
 	<dl dfn-type=value dfn-for=overflow-clip-margin>
 		: <dfn><<length [0,∞]>></dfn>
 		::
 			If the element does not have ''overflow: clip''
 			(or another property defined to respect this property),
 			this property has no effect.
 
-			Otherwise, dictates how much the clipping shape is expanded
-			from its initial shape.
-			The clipping shape is expanded outward in all directions
-			by the specified <<length>>,
+			Otherwise, the specified length dictates
+			how much the [=overflow clip edge=] is expanded from the [=padding edge=],
 			in exactly the same way as a 'box-shadow' grows
 			due to a positive spread distance
 			(including the effect on rounded corners).