<div class="button">
    <span>立即下载</span>
</div>

.button {
    display: inline-block;
    width: 200px;
    height: 46px;
    line-height: 46px;
    color: #ffffff;
    font-size: 18px;
    border-radius: 27px;
    text-align: center;
    background-image: linear-gradient(to right, #ff3300 0%, #eb4402 25%, #ffc404 50%, #eb4402 75%, #ff3300 100%);
    background-size: 200%;
    animation: colorGradient 1.5s infinite;
}

@keyframes colorGradient {
    0% {
        background-position: 0 0;
    }
    100% {
        background-position: 100% 0;
    }
}