[html] How to resize an image to fit in the browser window?

CSS3 introduces new units that are measured relative to the viewport, which is the window in this case. These are vh and vw, which measure viewport height and width, respectively. Here is a simple CSS only solution:

img {
    max-width: 100%;
    max-height: 100vh;
    height: auto;
}

The one caveat to this is that it only works if there are no other elements contributing height on the page.