//HTML
<section>
  <div>
  	<p>
  		Some Text
    </p>
  </div>
</section>

<button id="toggle-button">Click Me</button>

//CSS
div {
    overflow:hidden;
    transition:height 0.3s ease-out;
    height:auto;
  }

//JS
function collapseSection(element) {
  // get the height of the element's inner content, regardless of its actual size
  var sectionHeight = element.scrollHeight;
  
  // temporarily disable all css transitions
  var elementTransition = element.style.transition;
  element.style.transition = '';
  
  // on the next frame (as soon as the previous style change has taken effect),
  // explicitly set the element's height to its current pixel height, so we 
  // aren't transitioning out of 'auto'
  requestAnimationFrame(function() {
    element.style.height = sectionHeight + 'px';
    element.style.transition = elementTransition;
    
    // on the next frame (as soon as the previous style change has taken effect),
    // have the element transition to height: 0
    requestAnimationFrame(function() {
      element.style.height = 0 + 'px';
    });
  });
  
  // mark the section as "currently collapsed"
  element.setAttribute('data-collapsed', 'true');
}

function expandSection(element) {
  // get the height of the element's inner content, regardless of its actual size
  var sectionHeight = element.scrollHeight;
  
  // have the element transition to the height of its inner content
  element.style.height = sectionHeight + 'px';

  // when the next css transition finishes (which should be the one we just triggered)
  element.addEventListener('transitionend', function(e) {
    // remove this event listener so it only gets triggered once
    element.removeEventListener('transitionend', arguments.callee);
    
    // remove "height" from the element's inline styles, so it can return to its initial value
    element.style.height = null;
  });
  
  // mark the section as "currently not collapsed"
  element.setAttribute('data-collapsed', 'false');
}


document.querySelector('#toggle-button').addEventListener('click', function(e) {
  var section = document.querySelector('div');
  var isCollapsed = section.getAttribute('data-collapsed') === 'true';
    
  if(isCollapsed) {
    expandSection(section)
    section.setAttribute('data-collapsed', 'false')
  } else {
    collapseSection(section)
  }
});