Minor keyframe clarifications (#57)

* Minor keyframe clarifications

Must be in ascending frame order
First/last keyframe values are held to start/end
of the animation.

* Keyframes can't have the same `t` value

docs/specs/properties.md

@@ -15,9 +15,13 @@ Their structure depends on whether it's animated or not:
 
 {schema_object:properties/base-keyframe}
 
+Keyframe arrays MUST be stored in order of strictly ascending `t` frame number. Two keyframes cannot have the same `t` value.
+
 If `h` is present and it's 1, you don't need `i` and `o`, as the property will keep the same value
 until the next keyframe.
 
+If the first keyframe occurs after the start of the animation, the initial property value will be from the first keyframe. Similarly if the last keyframe is before the end of the animation, the last keyframe value will be held until the end.
+
 <h3 id="easing-handle">Keyframe Easing</h3>
 
 Keyframe easing handles are objects with `x` and `y` attributes, which are numbers within 0 and 1.