HTMLSourceElement: свойство width

<picture id="img">
  <source
    srcset="landscape.png"
    media="(min-width: 1000px)"
    width="1000"
    height="400" />
  <source
    srcset="square.png"
    media="(min-width: 800px)"
    width="800"
    height="800" />
  <source
    srcset="portrait.png"
    media="(min-width: 600px)"
    width="600"
    height="800" />
  <img
    src="fallback.png"
    alt="Image used when the browser does not support the sources"
    width="500"
    height="400" />
</picture>

const img = document.getElementById("img");
const sources = img.querySelectorAll("source");
console.log(Array.from(sources).map((el) => el.width)); // Output: [1000, 800, 600]