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CSS

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HTML 

              
                <h1>Packery - animate item size, vanilla JS</h1>

<p>Click items to toggle size</p>

<div class="grid">
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
  <div class="grid-item">
    <div class="grid-item-content"></div>
  </div>
</div>
!

CSS 

              
                * { box-sizing: border-box; }

body { font-family: sans-serif; }

.grid {
  background: #EEE;
  max-width: 1200px;
}

.grid-item {
  float: left;
}

/* item is invisible, but used for layout */
.grid-item,
.grid-item-content {
  width: 120px;
  height: 120px;
}

/* grid-item-content is visible, and transitions size */
.grid-item-content {
  width: 120px;
  height: 120px;
  background: #C09;
  border: 2px solid hsla(0, 0%, 0%, 0.5);
  -webkit-transition: width 0.4s, height 0.4s;
          transition: width 0.4s, height 0.4s;
}

.grid-item:hover .grid-item-content {
  background: #C90;
  cursor: pointer;
}

/* both item and item content change size */
.grid-item.is-expanded,
.grid-item.is-expanded .grid-item-content {
  width: 360px;
  height: 240px;
}

.grid-item.is-expanded {
  z-index: 2;
}

.grid-item.is-expanded .grid-item-content {
  background: #0C9;
}
!

JS 

              
                // external js: packery.pkgd.js, classie.js

var grid = document.querySelector('.grid');
var pckry = new Packery( grid, {
  itemSelector: '.grid-item'
});

grid.addEventListener( 'click', function( event ) {
  // don't proceed if item content was not clicked on
  if ( !matchesSelector( event.target, '.grid-item-content' )  ) {
    return;
  }
  var itemElem = event.target.parentNode;
  var isExpanded = itemElem.classList.contains('is-expanded');
  itemElem.classList.toggle('is-expanded');
  if ( isExpanded ) {
    // if shrinking, shiftLayout
    pckry.shiftLayout();
  } else {
    // if expanding, fit it
    pckry.fit( itemElem );
  }
});
!
999px

Console