how to fix button over page in html code example

Example: add fixed button bottom

#fixed_button{
    position: fixed;
    z-index: 999;
    right: 0.75rem;
    bottom: 0.75rem;
    display: none;
    width: 2.625rem;
    height: 2.625rem;
    border-radius: 1.875rem;
    background: #4f3cda url("../images/up-arrow.png") no-repeat center 47%;
        background-size: auto;
    background-size: 1.125rem 1.125rem;
    text-indent: -9999px;
}