the post has unreadable font in Vivaldi

Might be a Vivaldi problem. It has "Computer Modern Sans" in Firefox which seems to be perfectly legible

I found Computer Modern Sans quite unpleasant to read. Disabling it and reading Mastodon Sans Serif was much more pleasant

Yup, quite an odd choice to set the typeface for big blocks of text to a display typeface as CM Sans

In Firefox, all text outside the headline post, including the name and username that begin the headline post, appears to be in some kind of shrunken state which ruins its appearance. Zooming in solves the problem and causes the letters to appear correct rather than grotesquely malformed.

I don't know how the site was allowed into production like this.

you are right, it seems to be more zoom issue than font issue (or combo since I don't really experience this elsewhere, which is why I mentioned it), if I zoom out to 90% it becomes normally readable, but at default 100% zoom it is malformed unreadable (btw. I use 225% Windows 10 scaling at 2560x1440)

I can attest that the same thing happens to other fonts.

I have the opposite problem from you, though; for me the font (outside of the focused post) is already being rendered too small and zooming out doesn't help. Zooming in to 110% fixes it (while making some pretty big changes to the layout of the webpage...).

I'm at 150% scaling, 3840x2160.