Just asking but what is this meant to do exactly
Since it looks like it's meant to be a background but it does nothing, so I would recommend using instead of #demo, do
html {
background: transparent url("https://img5.goodfon.com/wallpaper/big/6/91/devushka-fon-vzgliad-rozy.jpg") repeat 100% 100%;
background-size: cover;
background-repeat: no-repeat
}