W3C Validation of SRCSET

Question

W3C Validator is throwing an error on the following code:

Answer 1

This answers only for your update: The reason for this is, that the browser chooses one candidate from your srcset, the src attribute is used as a fallback. This means to get the larger candidate you need to also add it to your srcset:

I don't really know your image layout, but I doubt, that sizes is correct here. If this image is displayed fullwidth until 1420px your sizes attribute should look like this.

sizes="(min-width: 1420px) 1420px, 100vw"

or

sizes="(max-width: 1420px) 100vw, 1420px"

Additionally you should use more images. Note if you use the width descriptor the browser automatically handles the density for you (for retina devices). Use at least 640w, 980w/1024w, 1420w, 2048w.