@jasondoesstuff @pjrvs @rachelandrew but now it's:

.marquee div {
display: block;
width: 200%;
height: 30px;

position: absolute;
overflow: hidden;

animation: marquee 5s linear infinite;
}

.marquee span {
float: left;
width: 50%;
}

@keyframes marquee {
0% { left: 0; }
100% { left: -100%; }
}