title: Can we really fill anything we want in transition-property CSS?
body: <p>I just realized we can put any kind of value we want behind <code>transition-property</code> CSS parameter, because I just did it (not on purpose). Then I went to <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/transition-property" rel="nofollow noreferrer">see the MDN docs</a> and they we can fill any custom-value :</p>
  
  <pre><code>/* &lt;custom-ident&gt; values */
  transition-property: test_05;
  transition-property: -specific;
  transition-property: sliding-vertically;
  
  /* Multiple values */
  transition-property: test1, animation4;
  transition-property: all, height, all;
  transition-property: all, -moz-specific, sliding;
  </code></pre>
  
  <p>So I am wondering : as  neither <code>test1</code> nor <code>animation4</code> is a valid CSS property, what kind of values, if it is not a property, can be passed in <code>transition-property</code> ? A keyframe ? what else ?</p>
  
  <p>[EDIT]: I added the example given by MDN for multiple values, where some <code>transition-property</code> aren't filled with valid CSS properties.</p>