[html] Darken background image on hover

The simplest way to do it without adding an extra overlay element, or using two images, is to use the :before or :after selector.

.image {
    position: relative;
}
.image:hover:after {
    content: ""; 
    position: absolute;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.1);
    top: 0;
    left:0;
}

This will not work in older browsers of course; just say it degrades gracefully!