クオートあり

クオートなし
/* クオートあり */
.sample1 {
    width: 100px;
    height: 100px;
    background: red;
    margin: 10px 0;
    position: relative;
    -webkit-animation: 'sample1' 1s infinite alternate;
    -moz-animation: 'sample1' 1s infinite alternate;
}
@-webkit-keyframes 'sample1' {
    from { left: 0 }
    to { left: 500px }
}
@-moz-keyframes 'sample1' {
    from { left: 0 }
    to { left: 500px }
}

/* クオートなし */
.sample2 {
    width: 100px;
    height: 100px;
    background: red;
    position: relative;
    -webkit-animation: sample2 1s infinite alternate;
    -moz-animation: sample2 1s infinite alternate;
}
@-webkit-keyframes sample2 {
    from { left: 0 }
    to { left: 500px }
}
@-moz-keyframes sample2 {
    from { left: 0 }
    to { left: 500px }
}