body {
    display: flex;
    justify-content: center;

    margin-top: 50px;
}

button {
    position: relative;

    background: gray;
    color: white;
    padding: 10px 30px;
    border-radius: 10px;
    cursor: pointer;
    overflow: hidden;
}

button::before {
    content: "";
    position: absolute;
    inset: 0;
    scale: 0;
    transform-origin: bottom right;

    background: red;
    border-radius: 10px;

    transition: scale 0.3s;
}

button span {
    position: relative;
    font-size: 2rem;
}

button:hover::before {
    scale: 1;
    transform-origin: top left;
}