Specify the behaviour when the scrollSource does not have a CSS layout box. Fixes issue #3.

Author: theres-waldo

Overview.bs

@@ -24,6 +24,8 @@ urlPrefix: https://w3c.github.io/web-animations/; type: dfn; spec: web-animation
     text: start delay
     text: target effect end
     text: timeline
+urlPrefix: https://www.w3.org/TR/cssom-view-1/; type: dfn; spec: cssom-view-1
+    text: CSS layout box
 </pre>
 
 # Introduction # {#intro}
@@ -496,18 +498,21 @@ The <dfn>effective time range</dfn> of a {{ScrollTimeline}} is calculated as fol
 The [=current time=] of a {{ScrollTimeline}} is calculated
 as follows:
 
-1.  Let <var>current scroll offset</var> be the current scroll offset of {{scrollSource}}
+1.  If {{scrollSource}} does not currently have a [=CSS layout box=], return an unresolved
+    time value.
+
+2.  Otherwise, let <var>current scroll offset</var> be the current scroll offset of {{scrollSource}}
     in the direction specified by {{orientation}}.
 
-2.  If <var>current scroll offset</var> is less than {{startScrollOffset}}, return an unresolved
+3.  If <var>current scroll offset</var> is less than {{startScrollOffset}}, return an unresolved
     time value if {{fill}} is <code>none</code> or <code>forwards</code>, 
     or 0 otherwise.
 
-3.  If <var>current scroll offset</var> is greater than or equal to {{endScrollOffset}}, 
+4.  If <var>current scroll offset</var> is greater than or equal to {{endScrollOffset}}, 
     return an unresolved time value if {{fill}} is <code>none</code> or
     <code>backwards</code>, or the [=effective time range=] otherwise.
 
-4.  Return the result of evaluating the following expression:
+5.  Return the result of evaluating the following expression:
 
     <blockquote>
       <code>(<var>current scroll offset</var> - {{startScrollOffset}}) / ({{endScrollOffset}} - {{startScrollOffset}}) &times; [=effective time range=]</code>