I started to use BSS recently. Unfortunately I was unable to use "background-attachment: fixed;" as BSS would only accept "initial" or "inherit", but not "fixed" as a value.
I finally found the solution :
element.style {
background: url("nuage.jpg") center / cover no-repeat fixed;
}
"fixed" seems to be a value for the background property, but not for the background-attachment property
I tried it on 2 different designs and it works. ;)