HTML

img srcset

<img 
    src="http://placehold.jp/3840x3840.png"

    srcset="http://placehold.jp/3840x3840.png 3840w,
            http://placehold.jp/2048x2048.png 2048w,
            http://placehold.jp/1920x1920.png 1920w,
            http://placehold.jp/1200x1200.png 1200w,
            http://placehold.jp/1080x1080.png 1080w,
            http://placehold.jp/828x828.png 828w,
            http://placehold.jp/750x750.png 750w,
            http://placehold.jp/640x640.png 640w"

     sizes="(max-width: 480px) 100vw, 
            (max-width: 1024px) 50vw, 
            33vw"
/>

srcset

<img srcset="uswnt-480.jpg 480w, 
             uswnt-640.jpg 640w, 
             uswnt-960.jpg 960w,
             uswnt-1280.jpg 1280w" 
     sizes="(max-width: 400px) 100vw, 
            (max-width: 960px) 75vw, 
            640px" 
     src="uswnt-640.jpg" alt="USWNT World Cup victory">

srcset

<img srcset="crest-383.jpg 1.5x, crest-510.jpg 2x" src="crest-255.jpg" alt="USWNT crest" />