Comment by storborg

Viewport width can be obtained via CSS, e.g. by using a media query for every screen width:

    @media (min-width: 400px) {
      .thing {
        background-image: url(size-400.png);
      }
    }
    @media (min-width: 401px) {
      .thing {
        background-image: url(size-401.png);
      }
    }

You could combine this with pixel density for even more specificity.