Add note about DOM interfaces state and future extensions/deprecations

Author: Sylvain Galineau

css-animations/Overview.bs

@@ -872,6 +872,8 @@ DOM Interfaces</h2>
 
 	CSS animations are exposed to the CSSOM through a pair of new interfaces describing the keyframes.
 
+    Note: the interfaces defined below reflect the interoperable API available as of this level of the specification. Future levels may deprecate parts of this API and extend others.
+
 <h3 id="interface-cssrule">
 The <code>CSSRule</code> Interface</h3>
 

css-animations/Overview.html

@@ -1538,6 +1538,9 @@ <h2 class="heading settled" data-level="6" id="interface-dom"><span class="secno
    <p>CSS animations are exposed to the CSSOM through a pair of new interfaces describing the keyframes.</p>
 
 
+   <p class="note" role="note">Note: the interfaces defined below reflect the interoperable API available as of this level of the specification. Future levels may deprecate parts of this API and extend others.</p>
+
+
    <h3 class="heading settled" data-level="6.1" id="interface-cssrule"><span class="secno">6.1. </span><span class="content">
 The <code>CSSRule</code> Interface</span><a class="self-link" href="#interface-cssrule"></a></h3>