Edit in JSFiddle

<div class="box"></div>
.box
{
    width:200px;
    height:200px;
    background:rgba(0,0,255,0.5);
    -webkit-animation:movingbox 5s infinite;
    -moz-animation:movingbox 5s infinite;    
    -o-animation:movingbox 5s infinite;
}

@-webkit-keyframes movingbox
{
    0%
    {
        margin-left:90%;
    }
    
    50%
    {
        margin-left:10%;
    }
      
    100%
    {
        margin-left:90%;
    }
}

@-moz-keyframes movingbox
{
    0%
    {
        margin-left:90%;
    }
    
    50%
    {
        margin-left:10%;
    }
      
    100%
    {
        margin-left:90%;
    }
}

@-o-keyframes movingbox
{
    0%
    {
        margin-left:90%;
    }
    
    50%
    {
        margin-left:10%;
    }
      
    100%
    {
        margin-left:90%;
    }
}