[css-timing] Fix behavior of step timing functions at boundaries

Author: birtles

css-timing/Overview.bs

@@ -248,6 +248,31 @@ and [=before flag=] as follows:
 
      decrement <var>current step</var> by one.
 
+1.   If [=input progress value=] &ge; 0 and <var>current step</var> &lt; 0,
+     let <var>current step</var> be zero.
+
+1.   If [=input progress value=] &le; 1 and <var>current step</var> &gt;
+     [=steps=], let <var>current step</var> be [=steps=].
+
+     <div class=note>
+
+     Steps 4 and 5 in this procedure ensure that given an [=input progress
+     value=] in the range [0, 1], a step timing function does not produce an
+     [=output progress value=] outside that range.
+
+     For example, although mathematically we might expect that a step timing
+     function with a [=step position=] of <a value for="steps()">start</a>
+     would step up when the [=input progress value=] is 1, intuitively, when we
+     apply such a timing function to a forwards-filling animation, we expect
+     it to produce an [=output progress value=] of 1 as the animation fills
+     forwards.
+
+     A similar situation arises for a step timing function with a [=step
+     position=] of <a value for="steps()">end</a> when applied to an animation
+     during its delay phase.
+
+     </div>
+
 1.   The [=output progress value=] is <code><var>current step</var>
      / [=steps=]</code>.