Note: [...] it is strongly encouraged to specify dimensions using the width and height attributes or with CSS. Without specified dimensions, the image will likely render with 300x150 dimensions because sizes="auto" implies contain-intrinsic-size: 300px 150px
Auto-sizes just shipped behind the Experimental Web Platform Features flag in Chrome Canary. Which makes this a good time to try to explain the one weird thing about it. In short: in addition to requiring loading=lazy, sizes=auto also basically requires that <img> elements have width and height attributes.
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