Related tragedy: font-weight: 300 used on webpages. In combination with low contrast (the most absurd thing I have seen was #999 on white) it's completely unreadable. I have actually written a bookmarklet to fix it: https://paste.debian.net/1207934/

I also have (partially outdated) custom stylesheet that fixes that for websites I {used to,} visit often: https://jenda.hrach.eu/w/blackweb