Talk about the relationship to asynchronous scrolling

Author: theres-waldo

Overview.bs

@@ -49,6 +49,17 @@ The behavior of the CSS markup is described in terms of the programming interfac
 User-agents that do not support script may still implement the CSS markup
 provided it behaves as if the underlying programming interfaces were in place.
 
+## Relationship to asynchronous scrolling ## {#async-scrolling}
+
+Some user agents support scrolling that's asynchronous with respect to layout
+or script. This specification is intended to be compatible with such an
+architecture. 
+
+Specifically, this specification allows expressing scroll-linked effects in a
+way that does not require script to run each time the effect is sampled. User
+agents that support asynchronous scrolling are allowed (but not required) to
+sample such effects asynchronously as well.
+
 # Use cases # {#use-cases}
 
 <em>This section is non-normative</em>
@@ -642,6 +653,21 @@ author is asking for trouble). In normal situations, including - importantly -
 when scrolling happens in response to input events, the rendered scroll offset
 and the state of scroll-driven animations will be consistent in each frame.
 
+User agents that composite frames asynchronously with respect to layout and/or
+script may, at their discretion, sample scroll-driven animations once per 
+<em>composited</em> frame, rather than (or in addition to) once per full layout 
+cycle. Again, if sampling such an animation causes a change to a scroll offset, 
+the animation will not be re-sampled to reflect the new offset until the next 
+frame.
+
+Nothing in this section is intended to require that scrolling block on layout
+or script. If a user agent normally composites frames where scrolling has
+occurred but the consequences of scrolling have not been fully propagated in
+layout or script (for example, <pre>scroll</pre> event listeners have not yet 
+run), the user agent may likewise choose not to sample scroll-driven animations
+for that composited frame. In such cases, the rendered scroll offset and the
+state of a scroll-driven animation may be inconsistent in the composited frame.
+
 # Scroll-triggered (but time-driven) animations # {#scroll-triggered-animations}
 
 An earlier draft of this proposal also provided for animations whose progress