demo.css

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    background: #eee;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen, Ubuntu,
        Cantarell, "Open Sans", "Helvetica Neue", sans-serif;
}

* {
    box-sizing: border-box;
}

input {
    border: none;
    outline: none;
    padding: 4px;
    border: 1px solid #aaa;
}

#wrapper {
    min-width: min(50%, 600px);
    padding: 1em 4em;
    background: #fff;
    border: 1px solid #ccc;
}

#wrapper > div {
    width: 100%;
    margin-bottom: 2vh;
    user-select: none;
}

#wrapper > div > label {
    font-weight: 500;
}

#wrapper > div > * {
    display: block;
    width: 100%;
    margin-bottom: 0.5em;
}

#custom_slider > div {
    position: relative;
    overflow: hidden;
    width: 50%;
    height: 2em;
    border-radius: 4px;
    padding: 0.25em;
    margin-top: 0.25em;
    background: #bbb;
    text-align: center;
    box-sizing: border-box;
}

#custom_slider > div * {
    position: absolute;
    padding: 0;
    margin: 0;
    top: 0;
    left: 0;
    height: 100%;
}

#custom_slider input {
    margin: 0;
    padding: 0;
    z-index: 1;
    border: none;
    outline: none;
    background: transparent;
    width: 100%;
    text-align: center;
}

#custom_slider span {
    width: 0;
    background: #999;
    pointer-events: none;
}


#custom_zone {
    display: block;
    width: 100%;
}

#custom_zone label {
    display: inline-block;
    padding-right: 1em;
}