HTML5 srcset: Mixing x and w syntax

1. Don't look what other browser do. Chrome is the only one doing it correctly (and FF 38+).
2. Don't look at this draft. It is not and won't be implemented. Here is the right one: https://html.spec.whatwg.org/multipage/embedded-content.html#attr-img-srcset

Mixing x with w in one descriptor is invalid and the browser will drop those candidates, because a w descriptor is always calculated into a x descriptor:

<!-- invalid -->
<img srcset="a.jpg 1x 300w, b.jpg 2x 600w" />

Mixing x with a w descriptor for different candidates is used/parsed by the browser but is invalid and doesn't make sense in 99% of all cases:

<!-- makes no sense: -->
<img srcset="300.jpg 1x, 600.jpg 600w" sizes="100vw" />

<!-- would make sense, because sizes is static in layoutpixel defined (i.e. 600 / 300 = 2x): -->
<img srcset="300.jpg 1x, 600.jpg 600w" sizes="300px" />

This means if you use the w descriptor you automatically also optimize for retina, you don't need to use an additional x descriptor (i.e. 480w 2x = 960w).

Additionally, in most cases of using a w descriptor your default/fallback image should also be defined in srcset:

<img src="small.png"
    srcset="small.png 480w, mediumg.png 640w, large.png 960w"
    sizes="100vw" />

3. try respimage polyfill (dilettantish try to advertise my polyfill)

What you want to do, can be achieved by the picture tag:

<picture>
  <source srcset="http://placehold.it/1400x600/e8117f/fff 1x, http://placehold.it/1400x600/e8117f/fff 2x" 
          media="(min-width: 1100px)" />
  <source srcset="http://placehold.it/700x300 1x, http://placehold.it/1400x600 2x" 
          media="(min-width: 720px)" />
  <source srcset="http://placehold.it/500x600/11e87f/fff 1x, http://placehold.it/1000x1200/11e87f/fff 2x" 
          media="(min-width: 450px)" />
  <img src="http://placehold.it/500x600/eee/ddd" 
       alt="image with artdirection" />
</picture>