style.css

html, body {
    height: 100%;
}

html {
    display: table;
    margin: auto;
}

body {
    display: table-cell;
    vertical-align: middle;
}

* {
    box-sizing: border-box;
    background-color: rgb(24,24,24);
}
#playerMaxHealth {
    background-color: rgb(24, 24, 24);
    position: relative;
    width: 100%;
    height: 50px;
    display: flex;
    justify-content: flex-end;
}

#playerHealth {
    background-color: rgb(0, 0, 64);
    position: absolute;
    width: 100%;
    height: 100%;
}

#enemyMaxHealth {
    background-color: rgb(24, 24, 24);
    display: block;
    position: relative;
    width: 100%;
    height: 50px;
}

#enemyHealth {
    background-color: rgb(64, 0, 0);
    position: absolute;
    width: 100%;
    height: 100%;
}

#container {
    position: relative;
    display: inline-block;
}

#HUD {
    display: flex;
    width: 100%;
    align-items: center;
}

ul {
    list-style-type: none;
}

body {
   display: inline-block; 
}