html {
    padding: 25vh 0 0;
}

body {
    max-width: 40em;
    margin: 0 auto;
}

.start-btn {
    color: #fff;
    cursor: pointer;
    border: none;
    position: relative;
    font-weight: bold;
    font-size: 14px;
    border-radius: 100px;
    text-decoration: none;
    padding: 10px 20px;
    text-transform: uppercase;
    background: linear-gradient(135deg, #98c87f, #19a0c2, #98c87f, #cccccc, #19a0c2);
    background-size: 400% 400%;
    display: inline-block;
    transition: all .5s ease;
    animation: gradient 8s cubic-bezier(0.6, -0.28, 0.74, 0.05) infinite;
}

.start-btn:hover {
    animation: gradient 8s ease infinite;
    color: #ffffff;
    box-shadow: 0 7px 18px 2px rgba(0 0 0 0.25);
}