srcset in webkit in xcode

Question

I'm trying to offer a choice of images in a HTML page in Webkit in Xcode. The following code shows the image in src, so the image is found, but the srcset just shows an empty box with the alt text. Have I missed something.

  <div>
    <img class="i1000"  src="cathedral porte sud 1500.jpg"/>
     <img class="i1000" srcset="cathedral porte sud 500.jpg 1x,
             cathedral porte sud 1000.jpg 2x,
             cathedral porte sud 1500.jpg 3x"
             alt="Becket"/>
      <p class="capt"> read from bottom</p>
  </div>

I've tested it on a iPhone 11 and iphone13 simulator.