This is a long-standing bug in Safari documented by many web developers, such as at https://github.com/ElMassimo/vite_ruby/issues/258 I would assume, given how many years people have been reporting this, that it will never be fixed and Safari will never be a reliably working browser.

Update: apparently it has been documented in some web projects that there is a long-standing bug in Safari where it will not allow you to ever clear the cache. https://github.com/ElMassimo/vite_ruby/issues/258 All we can do is block Safari users until Apple chooses not to publish broken software.

The above solution from @xophe no longer works as of Safari 17.5. Apple took care of that! I tried it and it is still using the old version of my CSS from its permanent, un-clearable memory cache. I am going to have my website block Safari users with a message suggesting they switch to a working browser. It's better than them looking at my site and thinking I don't know how to code at all because it's completely broken and I can't debug my CSS, which already works perfectly fine in every other browser, to work in Safari since it will never let me see my changes.

This is incredibly stupid. I've got a website that works fine in every other browser, and I can't debug it in Safari because Safari insists on using old CSS files no matter how many times I clear the cache. I'm getting ready to just block my site from Safari user-agents with a notice that Safari is broken and I don't support it. It's better than people looking at my site in Safari and thinking I don't know how to do web development.

100% agreed. I just want to take some simple notes, including a couple of inline images, and Apple has managed to make this a major headache. Both Notes.app and TextEdit.app both hang endlessly every 5 minutes. I just want to jot down some notes and I'm spending half my time for my computer to decide it's finally ready for the apparently high-bandwidth technologically advanced task of letting my type or paste a .png into a document. All third party solutions are either huge apps that are overkill for jotting down a note (Word) or are walled gardens that both cost money and store info in proprietary formats (Evernote, Devonthink) or both. Preview.app is incredibly slow and freezes randomly while trying to use it, too. Apple just can't get it right anymore. After 35 years as a Mac specialist, I am beginning not to see the point of sticking with an OS that doesn't even provide a way for me to jot down notes or save simple rich text documents without spending a half hour troubleshooting and then finally just giving up without a solution. Useless. MacOS no longer allows me to be productive, it eats my time instead of saving it.