How can I transition height: 0; to height: auto; u

Question

I am trying to make a <ul> slide down using CSS transitions.

The <ul> 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.
<hr>
<div id="parent0">
  <h1>Hover me (height: 0)</h1>
  <div id="child0">Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>
  </div>
</div>
<hr>
<div id="parent40">
  <h1>Hover me (height: 40)</h1>
  <div id="child40">Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>
  </div>
</div>

Answer 1

Here's a way to transition from any starting height, including 0, to auto (full size and flexible) without requiring hard-set code on a per-node basis or any user-code to initialize: https://github.com/csuwildcat/transition-auto. This is basically the holy grail for what you want, I believe --> http://codepen.io/csuwldcat/pen/kwsdF. Just slap the following JS file into your page, and all you need to do after that is add/remove a single boolean attribute - reveal="" - from the nodes you want to expand and contract.

Here's all you need to do as the user, once you include the code block found below the example code:

/*** Nothing out of the ordinary in your styles ***/
<style>
    div {
        height: 0;
        overflow: hidden;
        transition: height 1s;
    }
</style>

/*** Just add and remove one attribute and transition to/from auto! ***/

<div>
    I have tons of content and I am 0px in height you can't see me...
</div>

<div reveal>
     I have tons of content and I am 0px in height you can't see me...
     but now that you added the 'reveal' attribute, 
     I magically transitioned to full height!...
</div>

Here's the code block to include in your page, after that, it's all gravy:

Drop this JS file in your page - it all Just Works™

/* Code for height: auto; transitioning */

(function(doc){

/* feature detection for browsers that report different values for scrollHeight when an element's overflow is hidden vs visible (Firefox, IE) */
var test = doc.documentElement.appendChild(doc.createElement('x-reveal-test'));
    test.innerHTML = '-';
    test.style.cssText = 'display: block !important; height: 0px !important; padding: 0px !important; font-size: 0px !important; border-width: 0px !important; line-height: 1px !important; overflow: hidden !important;';
var scroll = test.scrollHeight || 2;
doc.documentElement.removeChild(test);

var loading = true,
    numReg = /^([0-9]*\.?[0-9]*)(.*)/,
    skipFrame = function(fn){
      requestAnimationFrame(function(){
        requestAnimationFrame(fn);
      });
    },
    /* 2 out of 3 uses of this function are purely to work around Chrome's catastrophically busted implementation of auto value CSS transitioning */
    revealFrame = function(el, state, height){
        el.setAttribute('reveal-transition', 'frame');
        el.style.height = height;
        skipFrame(function(){
            el.setAttribute('reveal-transition', state);
            el.style.height = '';
        });
    },
    transitionend = function(e){
      var node = e.target;
      if (node.hasAttribute('reveal')) {
        if (node.getAttribute('reveal-transition') == 'running') revealFrame(node, 'complete', '');
      } 
      else {
        node.removeAttribute('reveal-transition');
        node.style.height = '';
      }
    },
    animationstart = function(e){
      var node = e.target,
          name = e.animationName;   
      if (name == 'reveal' || name == 'unreveal') {

        if (loading) return revealFrame(node, 'complete', 'auto');

        var style = getComputedStyle(node),
            offset = (Number(style.paddingTop.match(numReg)[1])) +
                     (Number(style.paddingBottom.match(numReg)[1])) +
                     (Number(style.borderTopWidth.match(numReg)[1])) +
                     (Number(style.borderBottomWidth.match(numReg)[1]));

        if (name == 'reveal'){
          node.setAttribute('reveal-transition', 'running');
          node.style.height = node.scrollHeight - (offset / scroll) + 'px';
        }
        else {
            if (node.getAttribute('reveal-transition') == 'running') node.style.height = '';
            else revealFrame(node, 'running', node.scrollHeight - offset + 'px');
        }
      }
    };

doc.addEventListener('animationstart', animationstart, false);
doc.addEventListener('MSAnimationStart', animationstart, false);
doc.addEventListener('webkitAnimationStart', animationstart, false);
doc.addEventListener('transitionend', transitionend, false);
doc.addEventListener('MSTransitionEnd', transitionend, false);
doc.addEventListener('webkitTransitionEnd', transitionend, false);

/*
    Batshit readyState/DOMContentLoaded code to dance around Webkit/Chrome animation auto-run weirdness on initial page load.
    If they fixed their code, you could just check for if(doc.readyState != 'complete') in animationstart's if(loading) check
*/
if (document.readyState == 'complete') {
    skipFrame(function(){
        loading = false;
    });
}
else document.addEventListener('DOMContentLoaded', function(e){
    skipFrame(function(){
        loading = false;
    });
}, false);

/* Styles that allow for 'reveal' attribute triggers */
var styles = doc.createElement('style'),
    t = 'transition: none; ',
    au = 'animation: reveal 0.001s; ',
    ar = 'animation: unreveal 0.001s; ',
    clip = ' { from { opacity: 0; } to { opacity: 1; } }',
    r = 'keyframes reveal' + clip,
    u = 'keyframes unreveal' + clip;

styles.textContent = '[reveal] { -ms-'+ au + '-webkit-'+ au +'-moz-'+ au + au +'}' +
    '[reveal-transition="frame"] { -ms-' + t + '-webkit-' + t + '-moz-' + t + t + 'height: auto; }' +
    '[reveal-transition="complete"] { height: auto; }' +
    '[reveal-transition]:not([reveal]) { -webkit-'+ ar +'-moz-'+ ar + ar +'}' +
    '@-ms-' + r + '@-webkit-' + r + '@-moz-' + r + r +
    '@-ms-' + u +'@-webkit-' + u + '@-moz-' + u + u;

doc.querySelector('head').appendChild(styles);

})(document);

/* Code for DEMO */

    document.addEventListener('click', function(e){
      if (e.target.nodeName == 'BUTTON') {
        var next = e.target.nextElementSibling;
        next.hasAttribute('reveal') ? next.removeAttribute('reveal') : next.setAttribute('reveal', '');
      }
    }, false);

Answer 2

No hard coded values.

No JavaScript.

No approximations.

The trick is to use a hidden & duplicated div to get the browser to understand what 100% means.

This method is suitable whenever you're able to duplicate the DOM of the element you wish to animate.

.outer {
  border: dashed red 1px;
  position: relative;
}

.dummy {
  visibility: hidden;
}

.real {
  position: absolute;
  background: yellow;
  height: 0;
  transition: height 0.5s;
  overflow: hidden;
}

.outer:hover>.real {
  height: 100%;
}

Hover over the box below:
<div class="outer">
  <!-- The actual element that you'd like to animate -->
  <div class="real">
unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable
content unpredictable content unpredictable content unpredictable content
  </div>
  <!-- An exact copy of the element you'd like to animate. -->
  <div class="dummy" aria-hidden="true">
unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable
content unpredictable content unpredictable content unpredictable content
  </div>
</div>

Answer 3

As I post this there are over 30 answers already, but I feel my answer improves on the already accepted answer by jake.

I was not content with the issue that arises from simply using max-height and CSS3 transitions, since as many commenters noted, you have to set your max-height value very close to the actual height or you'll get a delay. See this JSFiddle for an example of that problem.

To get around this (while still using no JavaScript), I added another HTML element that transitions the transform: translateY CSS value.

This means both max-height and translateY are used: max-height allows the element to push down elements below it, while translateY gives the "instant" effect we want. The issue with max-height still exists, but its effect is lessened. This means you can set a much larger height for your max-height value and worry about it less.

The overall benefit is that on the transition back in (the collapse), the user sees the translateY animation immediately, so it doesn't really matter how long the max-height takes.

Solution as Fiddle

body {
  font-family: sans-serif;
}

.toggle {
  position: relative;
  border: 2px solid #333;
  border-radius: 3px;
  margin: 5px;
  width: 200px;
}

.toggle-header {
  margin: 0;
  padding: 10px;
  background-color: #333;
  color: white;
  text-align: center;
  cursor: pointer;
}

.toggle-height {
  background-color: tomato;
  overflow: hidden;
  transition: max-height .6s ease;
  max-height: 0;
}

.toggle:hover .toggle-height {
  max-height: 1000px;
}

.toggle-transform {
  padding: 5px;
  color: white;
  transition: transform .4s ease;
  transform: translateY(-100%);
}

.toggle:hover .toggle-transform {
  transform: translateY(0);
}

<div class="toggle">
  <div class="toggle-header">
    Toggle!
  </div>
  <div class="toggle-height">
    <div class="toggle-transform">
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
    </div>
  </div>
</div>

<div class="toggle">
  <div class="toggle-header">
    Toggle!
  </div>
  <div class="toggle-height">
    <div class="toggle-transform">
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
    </div>
  </div>
</div>

Answer 4

Ok, so I think I came up with a super simple answer... no max-height, uses relative positioning, works on li elements, & is pure CSS. I have not tested in anything but Firefox, though judging by the CSS, it should work on all browsers.

FIDDLE: http://jsfiddle.net/n5XfG/2596/

CSS

.wrap { overflow:hidden; }

.inner {
            margin-top:-100%;
    -webkit-transition:margin-top 500ms;
            transition:margin-top 500ms;
}

.inner.open { margin-top:0px; }

HTML

<div class="wrap">
    <div class="inner">Some Cool Content</div>
</div>

Answer 5

Use max-height in the transformation 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;
}

<div id="menu">
    <a>hover me</a>
    <ul id="list">
        <!-- Create a bunch, or not a bunch, of li's to see the timing. -->
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
    </ul>
</div>

Answer 6

You can't currently animate on height when one of the heights involved is auto, you have to set two explicit heights.