How can I transition height: 0; to height: auto; using CSS?

I am trying to make a

slide down using CSS transitions.




The

starts off at height: 0;. On hover, the height is set to height:auto;. However, this is causing it to simply appear, not transition,




If I do it from height: 40px; to height: auto;, then it will slide up to height: 0;, and then suddenly jump to the correct height.





How else could I do this without using JavaScript?









#child0 {
  height: 0;
  overflow: hidden;
  background-color: #dedede;
  -moz-transition: height 1s ease;
  -webkit-transition: height 1s ease;
  -o-transition: height 1s ease;

  transition: height 1s ease;
}
#parent0:hover #child0 {
  height: auto;
}
#child40 {
  height: 40px;
  overflow: hidden;
  background-color: #dedede;
  -moz-transition: height 1s ease;

  -webkit-transition: height 1s ease;
  -o-transition: height 1s ease;
  transition: height 1s ease;
}
#parent40:hover #child40 {
  height: auto;
}
h1 {
  font-weight: bold;
}



The only difference between the two snippets of CSS is one has height: 0, the other height: 40.


  
Hover me (height: 0)
  
Some content
    
Some content
    
Some content
    
Some content
    
Some content
    
Some content

    
  



  
Hover me (height: 40)
  
Some content
    
Some content
    
Some content
    
Some content

    
Some content
    
Some content
    
  

Answer

Use max-height in the transition and not height. And set a value on max-height to something bigger than your box will ever get.

See JSFiddle demo provided by Chris Jordan in another answer here.

#menu #list {
    max-height: 0;
    transition: max-height 0.15s ease-out;
    overflow: hidden;
    background: #d5d5d5;

}

#menu:hover #list {
    max-height: 500px;
    transition: max-height 0.25s ease-in;
}

    hover me
    

        

        
item
        
item
        
item
        
item
        
item