Individual collapsible areas with accessibility in mind
Everything looks great, and I can see what you are trying to achieve. 

Only things I can recommend is to clean your code, make it as simple as possible, to make it easy to read and come back on later. 

For example this line here:

while ((el = el.parentElement) &amp;&amp; !el.classList.contains(cls));

In the way it is used, it's okay, but it doesn't explain well what the intention is, instead, try to be more "if this, then", very straight forward. Instead of using a while you could use recursion instead, removing more confusion. 

Another thing is, you may want to expand your variable names. You probably will put this code through a minifier either way, so no need to keep things short. 

function findAncestor(element, findClass) {
   if (element.classList.contains(findClass)) {
      return element;
   }
   if (!element.parentElement) {
      // Did we hit body?
      return false;
   }
   // We didn't find it, so check the parent.
   return findAncestor(element.parentElement, findClass);
}


Now we can read this function, and know exactly whats happening, without making any assumptions. 

The next function is great, and works well, but we could make this more accessible using the [hidden] attribute along with a .hidden class, this is so we don't need to worry about compatibility. 

We only want to use the parent as a light wrapper, from that parent we can find the child we actually want to show/hide

You can do a bit of reading on [hidden] here
https://www.paciellogroup.com/blog/2012/05/html5-accessibility-chops-hidden-and-aria-hidden/ 

function openHolder() {
   var region = findAncestor(this, 'js-collapsible-region');
   var content = region.querySelector('.collapsible-area');

   if (content.classList.contains('hidden')) {
      content.classList.remove('hidden');
      content.removeAttribute('hidden');
   } else {
      content.classList.add('hidden');
      content.setAttribute('hidden', '');
   }
}


We also want to leave state out of the DOM if it isn't visual state (like we can check if something is hidden, but we could also track that another way).

We then simply need to add our even listeners on like we did before:

var btn = document.getElementsByClassName('js-collapsible-btn');
for (var i = 0; i &lt; btn.length; i++) {
    // add click event to open holder
    btn[i].addEventListener('click', openHolder, false);
}


We can also change this to a "functional" approach to remove the loop (which we needed to handle manually)

Array.from(document.getElementsByClassName('js-collapsible-btn'))
   .forEach(function(element) {
      element.addEventListener('click', openHolder, false);
   });


The last thing we should do, is instantly tell the screen reader is hidden but having the content marked as hidden to start with:

&lt;div class="js-collapsible-region"&gt;
    &lt;h2&gt;Title&lt;/h2&gt;
    &lt;p&gt;Content goes here&lt;/p&gt;
    &lt;div&gt;
        &lt;button class="js-collapsible-btn"&gt;Click to show some text&lt;/button&gt;
    &lt;/div&gt;
    &lt;div class="collapsible-area hidden" hidden&gt;
        &lt;p&gt;Suprise&lt;/p&gt;
    &lt;/div&gt;
&lt;/div&gt;

&lt;div class="js-collapsible-region"&gt;
    &lt;h2&gt;Title&lt;/h2&gt;
    &lt;p&gt;Content goes here&lt;/p&gt;
    &lt;div&gt;
        &lt;button class="js-collapsible-btn"&gt;Click to show some text&lt;/button&gt;
    &lt;/div&gt;
    &lt;div class="collapsible-area hidden" hidden&gt;
        &lt;p&gt;Suprise&lt;/p&gt;
    &lt;/div&gt;
&lt;/div&gt;


We also will need to change the styles to match 

.js-collapsible-region .collapsible-area {
    overflow: hidden;
}

.js-collapsible-region .collapsible-area.hidden {
    height: 0px !important;
}

.js-collapsible-region .collapsible-area:not(.hidden) {
    transition: height .3s linear;
}


You should note that the attribute hidden applies the styles display: none, which you may not want, so you can change your styles to override the way you want them to work. 

Here is the resulting codepen https://codepen.io/anon/pen/KqgmNY

For some fun, since we are using ES6 with classList anyway, have a look at this version https://codepen.io/anon/pen/mwrmpL