Voici [HTML Global Hidden Attribute](http://www.w3schools.com/tags/att_global_hidden.asp):

```html
<div class="comment-form" hidden>...</div>
```

```js
$(document).on('click', '.reply', function(e){
  $(this).prev().removeAttr('hidden')
});
```

Polyfill super clean:
```css
[hidden] { display: none } /* Pour IE 8/9 */
```