The srcset Attribute.

Below is a image (img) element with both a regular src attribute as well as a srcset attribute. There is a stylesheet that sets the dimensions of images to 400x400px. On browsers without srcset support, the value of the src attribute will be used as the image src [default image]. On regular resolution displays, the 1x variant of the srcset will be used [1x image]. On displays with 2 device pixels per CSS pixel, the 2x variant of the srcset will be used [2x image]. Similarly, there is a 3x image, and a 4x image.

Example of the srcset attribute. Image contains a coloured striped pattern with some inline text that indicates which of the candidate images were selected.

The HTML for the above image is: 

<img src="image-src.png" srcset="image-1x.png 1x, image-2x.png 2x,
                                 image-3x.png 3x, image-4x.png 4x">