If you’re not on a high DPI display, then all I can think is that some font hinting/autoaliasing settings must be out of wack.

All the screenshots look blurry to me.

Btw, if you are upscaling XWayland windows, have you tried disabling that and then using a higher font size in Emacs?

I’ve always used (shr-render-buffer (current-buffer)). I don’t know the differences from eww-display-html, but one is that it pops up a buffer named *html* and renders it there.