The image below is loaded with the following img tag.

<img
  class="srcset-demo-img"
  alt=""
  src="images/2000x1000.png"

  srcset="
    images/500x250.png   500w,
    images/1000x500.png  1000w,
    images/1500x750.png  1500w,
    images/2000x1000.png 2000w,
    images/3000x1500.png 3000w">

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This code is on GitHub at https://github.com/salcode/srcset-demo