Edit in JSFiddle

<div class="scrollable">
    <div class="items">
        <img src="http://placehold.it/333x500/E8117F/FFFFFF&text=Horizontal"/>
        <img src="http://placehold.it/333x500/FFFFFF/E8117F&text=css3"/>
        <img src="http://placehold.it/333x500/3D668F/FFFFFF&text=slide show"/>
    </div>
</div>

body {
    padding: 1em;
    background: #999
}
.scrollable {
    width: 333px;
    margin: 0 auto;
    padding: 0;
    border:10px solid #fff;
    background: #000;
    position: relative;
    overflow: hidden;
    text-align: center;
}
img {
    max-width: 333px;
    margin: 0;
    float:left;
}
.items {
    width:999px;
    -webkit-animation: hscroll 12s infinite;
    -moz-animation: hscroll 12s infinite;
    -ms-animation: hscroll 12s infinite;
}

@-webkit-keyframes hscroll {
  0%   { margin-left: 0; }
27.33%  { margin-left: 0 }
  33.33%  { margin-left: -333px; }
60.66% { margin-left: -333px; }
66.66% { margin-left: -666px; }
94.99% { margin-left: -666px; }
  100%  { margin-left: 0 }
}

@-moz-keyframes hscroll {
  0%   { margin-left: 0; }
27.33%  { margin-left: 0 }
  33.33%  { margin-left: -333px; }
60.66% { margin-left: -333px; }
66.66% { margin-left: -666px; }
94.99% { margin-left: -666px; }
  100%  { margin-left: 0 }
}

@-ms-keyframes hscroll {
  0%   { margin-left: 0; }
27.33%  { margin-left: 0 }
  33.33%  { margin-left: -333px; }
60.66% { margin-left: -333px; }
66.66% { margin-left: -666px; }
94.99% { margin-left: -666px; }
  100%  { margin-left: 0 }
}