[IMP] playground: improve code samples

closes #126

doc/animations.md

@@ -2,7 +2,7 @@
 
 
 Animation is a complex topic. There are many different use cases, and many
-solutions and technologies.
+solutions and technologies. Owl only supports some basic use cases.
 
 ## Simple CSS effects
 
@@ -12,12 +12,16 @@ example:
 
 
 ```xml
-<button class="flash" t-on-click="doSomething">Click</button>
+<a class="btn flash" t-on-click="doSomething">Click</a>
 ```
 
 with the following CSS:
 
 ```css
+btn {
+    background-color: gray;
+}
+
 .flash {
   transition: background 0.5s;
 }
@@ -37,3 +41,4 @@ A more complex situation occurs when we want to transition an element in or out
 of the page.  For example, we may want a fade-in and fade-out effect.
 
 The `t-transition` directive is here to help us (see [QWeb documentation](qweb.md#t-transition-directive)).
+