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CSS

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JavaScript

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HTML 

              
                <h1>Masonry - prepended, vanilla JS</h1>

<p><button class="prepend-button">Prepend new items</button></p>

<div class="grid">
  <div class="grid-item grid-item--width3"></div>
  <div class="grid-item grid-item--height3"></div>
  <div class="grid-item"></div>
  <div class="grid-item grid-item--height2"></div>
</div>
!

CSS 

              
                * { box-sizing: border-box; }

body { font-family: sans-serif; }

/* ---- grid ---- */

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

/* clearfix */
.grid:after {
  content: '';
  display: block;
  clear: both;
}

/* ---- grid-item ---- */

.grid-item {
  width: 160px;
  height: 120px;
  float: left;
  background: #D26;
  border: 2px solid #333;
  border-color: hsla(0, 0%, 0%, 0.5);
  border-radius: 5px;
}

.grid-item--width2 { width: 320px; }
.grid-item--width3 { width: 480px; }
.grid-item--width4 { width: 640px; }

.grid-item--height2 { height: 200px; }
.grid-item--height3 { height: 260px; }
.grid-item--height4 { height: 360px; }

button {
  font-size: 20px;
}
!

JS 

              
                // external js: masonry.pkgd.js

var grid = document.querySelector('.grid');
var msnry = new Masonry( grid, {
  columnWidth: 160,
  itemSelector: '.grid-item'
});

var prependButton = document.querySelector('.prepend-button');
prependButton.addEventListener( 'click', function() {
  // create new item elements
  var elems = [];
  var fragment = document.createDocumentFragment();
  for ( var i = 0; i < 3; i++ ) {
    var elem = getItemElement();
    fragment.appendChild( elem );
    elems.push( elem );
  }
  // append elements to container
  grid.insertBefore( fragment, grid.firstChild );
  // add and lay out newly appended elements
  msnry.prepended( elems );
});

// create <div class="grid-item"></div>
function getItemElement() {
  var elem = document.createElement('div');
  var wRand = Math.random();
  var hRand = Math.random();
  var widthClass = wRand > 0.8 ? 'grid-item--width3' : wRand > 0.6 ? 'grid-item--width2' : '';
  var heightClass = hRand > 0.85 ? 'grid-item--height4' : hRand > 0.6 ? 'grid-item--height3' : hRand > 0.35 ? 'grid-item--height2' : '';
  elem.className = 'grid-item ' + widthClass + ' ' + heightClass;
  return elem;
}
!
999px

Console