warum eigentlich nicht so:
<img class="" src="/assets/css/images/banner_1920.png"
srcset=" /assets/css/images/banner_640.png 480w,
/assets/css/images/banner_640.png 768w,
/assets/css/images/banner_1280.png 992w,
/assets/css/images/banner_1920.png 1080w"
sizes=" (min-width: 50em) 33vw...