[css-transitions-2] Add section of transition-timing-function and getComputedTiming (issue #4288) (#4306)

Author: george-steel

css-transitions-2/Overview.bs

@@ -80,6 +80,17 @@ sub-property that is animatable. The <dfn>expanded transition property
 name</dfn> of a transition is the name of the longhand sub-property for which
 the transition was generated (e.g. 'border-left-width').
 
+## The transition timing function ## {#transition-timing-function}
+
+When creating a {{KeyframeEffect}} for a transition, the effect's {{EffectTiming/easing}}
+option is set to the value of the matching 'transition-timing-function' property,
+which must be used by {{AnimationEffect/getComputedTiming()}}.
+
+Note: Unlike CSS animations, the timing function for CSS transitions applies to
+the animation effect as opposed to the individual keyframes since this allows
+it to be reflected in the <a>transformed progress</a> as used when calculating
+the reversing shortening factor.
+
 ## Owning element ## {#owning-element-section}
 
 The <dfn>owning element</dfn> of a transition refers to the element or

web-animations-1/Overview.bs

@@ -2980,7 +2980,7 @@ function</a>.
 
 <h4 id="calculating-the-transformed-progress">Calculating the transformed progress</h4>
 
-The <dfn>transformed progress</dfn> is calculated from the
+The <dfn export>transformed progress</dfn> is calculated from the
 <a>directed progress</a> using the following steps:
 
 1.  If the <a>directed progress</a> is <a>unresolved</a>,