html,
body {
    width: 100%;
    height: 100%;
    display: flex;
    overflow: hidden;
}

.button {
    position: fixed;
    top: 0;
    left: 0;
    width: 120px;
    height: 64px;
    line-height: 64px;
    text-align: center;
    border: 1px solid #eee;
    cursor: pointer;
    transition: .3s all;
    z-index: 10;

    &:hover {
        background: #eee;
    }
}

p {
    margin: 50px auto;
    width: 500px;
    height: 250px;
    font-size: 24px;
    line-height: 1.5;
    color: #666;
    overflow: hidden;
}

a {    
    background: 
        linear-gradient(90deg, #ff5722, #ff5722),
        linear-gradient(90deg, #aaa, #aaa);
    // background: linear-gradient(90deg, #ff3c41, #fc0, #8500d8);
    background-size: 100% 100%, 0 100px;
    // background-size: 100% 100%;
    background-repeat: no-repeat;
    background-position: 100% 100%, 0 100%;
    // background-position: 100% 100%;
    cursor: pointer;
    color: transparent;
    background-clip: text;
    line-height: 3;
    opacity: 0;
}
a::before {
    content: "";
    position: fixed;
    background: rgba(0, 0, 0, .8);
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -1;
    transition: .3s all linear;
    opacity: 0;
}
.button:hover ~ p a {
    transition: 1.2s background .3s ease-out, .8s line-height ease-out, .6s opacity ease-in;
    background-size: 0 100px, 100% 100%;
    // background-position: 0 100%, 0 100%;
    // background-size: 0 100px;
    color: transparent;
    line-height: 1;
    opacity: 1;
}
.button:hover ~ p a::before {
    opacity: 1;
}