html - CSS3 transition doesn't work with display property

Question

This question already has answers here:

Answer 1

display:none; removes a block from the page as if it were never there. A block cannot be partially displayed; it’s either there or it’s not. The same is true for visibility; you can’t expect a block to be half hidden which, by definition, would be visible! Fortunately, you can use opacity for fading effects instead.
- reference

As an alternatiive CSS solution, you could play with opacity, height and padding properties to achieve the desirable effect:

#header #button:hover > .content {
    opacity:1;
    height: 150px;
    padding: 8px;    
}

#header #button .content {
    opacity:0;
    height: 0;
    padding: 0 8px;
    overflow: hidden;
    transition: all .3s ease .15s;
}

(Vendor prefixes omitted due to brevity.)

Here is a working demo. Also here is a similar topic on SO.

#header #button {
  width:200px;
  background:#ddd;
  transition: border-radius .3s ease .15s;
}

#header #button:hover, #header #button > .content {
    border-radius: 0px 0px 7px 7px;
}

#header #button:hover > .content {
  opacity: 1;
  height: 150px;
  padding: 8px;    
}

#header #button > .content {
  opacity:0;
  clear: both;
  height: 0;
  padding: 0 8px;
  overflow: hidden;

  -webkit-transition: all .3s ease .15s;
  -moz-transition: all .3s ease .15s;
  -o-transition: all .3s ease .15s;
  -ms-transition: all .3s ease .15s;
  transition: all .3s ease .15s;

  border: 1px solid #ddd;

  -webkit-box-shadow: 0px 2px 2px #ddd;
  -moz-box-shadow: 0px 2px 2px #ddd;
  box-shadow: 0px 2px 2px #ddd;
  background: #FFF;
}

#button > span { display: inline-block; padding: .5em 1em }

<div id="header">
  <div id="button"> <span>This is a Button</span>
    <div class="content">
      This is the Hidden Div
    </div>
  </div>
</div>