[css-sizing] Specify that sizing properties containing %s are treated…

… as property's initial value for intrinsic size contribution. Fixes w3c#1132.

css-sizing-3/Overview.bs

@@ -839,6 +839,16 @@ Intrinsic Contributions</h3>
 	that contains only that box,
 	if that hypothetical float's containing block is zero-sized/infinitely-sized.
 
+	If the box is [=non-replaced=],
+	then the value of any [=sizing property=] specified on it
+	as an expression containing a percentage
+	(such as ''10%'' or ''calc(10px + 0%)'')
+	is treated <em>for the purpose of calculating the box’s intrinsic size contribution only</em>
+	as that property’s initial value.
+	For example, given a box with ''width: calc(20px + 50%)'',
+	its max-content contribution is calculated as if its 'width' were ''width/auto''.
+	The percentage is honored as usual, however, during the actual sizing of the box itself.
+
 	However, in the case of a [=replaced element|replaced=] box with a percentage-based 'width'/'max-width'/'height'/'max-height',
 	the percentage is resolved to zero
 	when calculating the <a>min-content contribution</a> in the corresponding axis.