Sorry, that should be '2x', not 'x2'. The img '2x-res' property could also be used to provide an asset-specific 2x-res filename key as such:

  <img 2x-res="@2x" />

If we wanted this to affect the image asset requested by the CSS background-image and border-image values as well, then a new CSS property would be required for flagging 2x-res active or not and an asset-specific 2x filename key. But its use would be vastly superior to -webkit-image-set, as you could just do this:

  .class {2x-res: "@2x"}

or:

  .class {2x-res: "false"}

The only limitation is that these 2x assets must share the same filename with the 1x, except for the addition of a 2x key at the end of the filename. However, that seems to be what people are already doing simply to keep track of their assets, and it's much easier to sell me on this imposition than on having to redo all my CSS in the most redundant and painful way imaginable.