The article mentioned the problem with this approach: Your browser loads the HTML, CSS, JS simultaneously with the images on the start of the page load - so when you find out what size you need at runtime (via JS), and then put in the right img-src, it will be slower than loading a higher-res version right from the start.

Another option would be to "guess" the right size to serve based on the User Agent String (maybe you meant this and I misunderstood?). This could work, although the server may very well guess wrong.