[seanhunter]

The article says

  The final <img> tag also specifies common attributes like width, height and alt text. Those attributes will apply regardless of which source the browser chooses!

The official spec for the picture element is here https://html.spec.whatwg.org/multipage/embedded-content.html... and it says

  The picture element is a container which provides multiple sources to its contained img element to allow authors to declaratively control or give hints to the user agent about which image resource to use, based on the screen pixel density, viewport size, image format, and other factors. It represents its children.

So I read that as the alt text (as well as other attributes other than the actual src url) come from the img tag