[css-easing-2][editorial] Add a note about handling infinite progress values. w3c#8344

Author: tabatkins

css-easing-2/Overview.bs

@@ -115,6 +115,13 @@ may be outside the range [0, 1].
 The <dfn>output progress value</dfn> is a real number in the
 range [-&infin;, &infin;].
 
+Note: While CSS numbers have a theoretically infinite range
+(see [[css-values-4#numeric-ranges]])
+UAs will automatically clamp enormous numbers to a reasonable range.
+If easing functions are used outside of the CSS context,
+care must be taken to either correctly handle potential infinities,
+or clamp the [=output progress value=].
+
 Some types of easing functions also take an additional boolean [=before flag=]
 input which is defined subsequently.