glass_button.css

body {
    margin: 0;
    padding: 0;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background: linear-gradient(-30deg, #1ec4e9 0%, #673ab7 50%, #262626 50%, #607d8b 100%);
}

a {
    position: relative;
    padding: 25px 50px;
    text-decoration: none;
    color: #fff;
    font-size: 2em;
    text-transform: uppercase;
    font-family: sans-serif;
    letter-spacing: 4px;
    overflow: hidden;
    background: rgba(255, 255, 255, .1);
    box-shadow: 0 5px 5px rgba(0, 0, 0, .2);
}

a:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: linear-gradient();
}

a:after {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: linear-gradient(90deg, transparent, rgba(255, 255, 255, .4), transparent);
    transition: 0.5s;
    transition-delay: 0.1s;
}

a:hover:after {
    left: 100%;
}

a span {
    position: absolute;
    display: block;
    transition: 0.5s ease;
}

a span:nth-child(1) {
    top: 0;
    left: 0;
    width: 0;
    height: 1px;
    background: #fff;
}

a:hover span:nth-child(1) {
    width: 100%;
    transform: translateX(100%);
}

a span:nth-child(3) {
    bottom: 0;
    right: 0;
    width: 0;
    height: 1px;
    background: #fff;
}

a:hover span:nth-child(3) {
    width: 100%;
    transform: translateX(-100%);
}

a span:nth-child(2) {
    top: 0;
    left: 0;
    width: 1px;
    height: 0;
    background: #fff;
}

a:hover span:nth-child(2) {
    height: 100%;
    transform: translateY(100%);
}

a span:nth-child(4) {
    bottom: 0;
    right: 0;
    width: 1px;
    height: 0;
    background: #fff;
}

a:hover span:nth-child(4) {
    height: 100%;
    transform: translateY(-100%);
}