[css-animations-2] Make animation-duration:auto resolve to 0s for leg…

…acy compat #6530

css-animations-2/Overview.bs

@@ -462,6 +462,12 @@ Declaring Animations</h2>
   However, if 'animation-fill-mode' is set to ''animation-fill-mode/none''
   the keyframes of the animation animation will have no noticeable effect.
 
+  For backwards-compatibility with Level 1,
+  when the [=computed value=] of 'animation-timeline' is ''animation-timeline/auto''
+  (i.e. only one list value, and that value being ''animation-timeline/auto''),
+  the [=resolved value=] of ''animation-duration/auto'' for 'animation-duration'
+  is ''0s'' whenever its [=used value=] would also be ''0s''.
+
 ## The 'animation-timing-function' property ## {#animation-timing-function}
 
 The 'animation-timing-function' is used to determine the [=timing function=]