I tried simple thing

#container-top-left {
    border-right: 1px #4f4f4f solid;
    border-bottom: 1px #4f4f4f solid;
    background-image: url("../images/icon-off.png");
    background-position: 59px 59px;
}
#container-top-left.on {
    background-image: url("../images/icon-on.png");
}

doesn’t work. Why?

Even this doesnt work:

a:active#container-top-left {
    background-image: url("../images/icon-on.png");
}

Can I read somewhere what does or doesn’t work for web app/widget? In documentation it says html5/CSS3 but it is not true.

Is it a bug? Does it happen to anyone?

Thanks.

edit: I miss-copied a block. Fixed.