To actually see the small "Data URLs" images upgrade to normal large images in action on the lower grid, change the throttling to 'Regular 3G' in the 'Network' tab.
Change the width of the browser to see the React-Hover-Grids resize and adjust the images via css. The NPM package has more information.
This Hover-Grid has no Javascript. The top grid is the low res SSR delivered grid with data URI images inside css, whilst below is the same grid after high rez images have loaded over the fuzzy pictures.

Screen sizes supported in [412, 414, 480, 568, 640, 667, 720, 732, 736, 768, 800, 900, 1024, 1050, 1080, 1200, 1280, 1360, 1366, 1440, 1600, 1680, 1920]