[css-transitions] Partially define when computed values change by saying that computed values have to change before the change is presented/displayed/used.

Addresses part of https://www.w3.org/Bugs/Public/show_bug.cgi?id=16016

Author: dbaron

css-transitions/Overview.bs

@@ -575,7 +575,15 @@ url: http://w3c.github.io/dom/#constructing-events; type: dfn; text: event const
         changes to which selectors match elements,
         changes to style sheets or style attributes,
         and other things.
-        This specification does not define when computed styles are updated.
+        This specification does not define when computed styles are updated,
+        beyond saying that implementations must not
+        use, present, or display something resulting from the CSS
+        cascading, value computation, and inheritance process [[!CSS3CASCADE]]
+        without updating the computed style
+        (which means merely that implementations cannot avoid
+        meeting requirements of this specification
+        by claiming not to have updated the computed style
+        as part of handling a style change).
         However,
         when an implementation updates the computed style for an element
         to reflect one of these changes,
@@ -1599,6 +1607,7 @@ dated 19 November 2013</a>:</p>
   <li>The interpolation of ''shadow/inset'' values on shadow lists is no longer backwards.</li>
   <li>A [[#conformance]] section and [[#idl-index]] have been added</li>
   <li>The identifiers accepted by 'transition-property' are defined in terms of <<custom-ident>>.</li>
+  <li>Define a little bit more about when changes to computed style happen, by saying at least that implementations must not update the effects of computed style without actually updating computed style.</li>
 </ul>
 
 <p>For more details on these changes, see the version control <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log since 2015 January 26</a> and the <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.src.html">change log from 2013 March 28 to 2015 January 26</a>.</p>

css-transitions/Overview.html

@@ -965,7 +965,15 @@ <h2 class="heading settled" data-level="3" id="starting"><span class="secno">3.
         changes to which selectors match elements,
         changes to style sheets or style attributes,
         and other things.
-        This specification does not define when computed styles are updated.
+        This specification does not define when computed styles are updated,
+        beyond saying that implementations must not
+        use, present, or display something resulting from the CSS
+        cascading, value computation, and inheritance process <a data-link-type="biblio" href="#biblio-css3cascade">[CSS3CASCADE]</a>
+        without updating the computed style
+        (which means merely that implementations cannot avoid
+        meeting requirements of this specification
+        by claiming not to have updated the computed style
+        as part of handling a style change).
         However,
         when an implementation updates the computed style for an element
         to reflect one of these changes,
@@ -2426,6 +2434,8 @@ <h2 class="heading settled" data-level="9" id="changes"><span class="secno">9. <
 Conformance</a> section and <a href="#idl-index">IDL Index</a> have been added
 
     <li>The identifiers accepted by <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a> are defined in terms of <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#identifier-value">&lt;custom-ident></a>.
+  
+    <li>Define a little bit more about when changes to computed style happen, by saying at least that implementations must not update the effects of computed style without actually updating computed style.
 </ul>