[css-position-3] Clarify that value may become negative. <https://lis…

…ts.w3.org/Archives/Public/www-style/2020Apr/0021.html>

css-position-3/Overview.bs

@@ -598,6 +598,7 @@ Sticky positioning</h3>
 	If this results in a [=sticky view rectangle=] size in any axis
 	less than the size of the [=border box=] of the [=sticky=] box in that axis,
 	then the effective end-edge inset in the affected axis is reduced
+	(possibly becoming negative)
 	to bring the [=sticky view rectangle=]’s size up to
 	the size of the [=border box=] in that axis.
 
@@ -701,7 +702,8 @@ Absolute Positioning</h3>
 	If these adjustments result in
 	an effective [=containing block=] size in any axis less than zero,
 	then the [=weaker inset=] in the affected axis
-	is reduced to bring that size up to zero.
+	is reduced (possibly becoming negative)
+	to bring that size up to zero.
 	In the case that only one inset is ''auto'',
 	that is the <dfn>weaker inset</dfn>;
 	otherwise the [=weaker inset=] is the end-edge inset.