[css-grid-3] Add masonry-slack:infinite. w3c#10883

Author: tabatkins

css-grid-3/Overview.bs

@@ -1012,7 +1012,7 @@ Placement Precision: the 'masonry-slack' property</h3>
 
 	<pre class=propdef>
 	Name: masonry-slack
-	Value: <<length-percentage>>
+	Value: <<length-percentage>> | infinite
 	Initial: 1em
 	Percentages: relative to the [=grid-axis=] [=content box=] size of the [=masonry container=]
 	Inherited: no
@@ -1035,7 +1035,7 @@ Placement Precision: the 'masonry-slack' property</h3>
 	causing them to fill in order.
 
 	<dl dfn-type=value dfn-for=masonry-slack>
-		: <dfn><<length>></dfn>
+		: <dfn><<length-percentage>></dfn>
 		:: Specifies the <dfn dfn for=masonry>tie threshold</dfn>
 			for the [=masonry container=].
 			Placement positions are considered to be equally good (“tied”)
@@ -1044,6 +1044,19 @@ Placement Precision: the 'masonry-slack' property</h3>
 
 			Note: The initial value is a “small” distance (''1em'')
 			that is probably appropriate to represent “close enough”.
+
+		: <dfn>infinite</dfn>
+		:: Specifies an infinite [=tie threshold=].
+			This makes items distribute themselves strictly in order,
+			without considering the length of the tracks at all.
+
+			Note: This value can result in consecutive items being placed
+			in dramatically different positions in the [=stacking axis=],
+			which can be confusing to readers.
+			If the initial value (`1em`) is too small,
+			consider a larger value (such as `10em` or `50vh`)
+			instead of `infinite`.
+
 	</dl>
 
 	Issue: Is ''1em'' the right default?