Need Help About Cross Browser Compatibility

Dear All

I need some help about cross browser compatibility for my website -> https://angora.me/blog/

And here is my CSS code ->
https://angora.me/style.css

On modern browser, my website looks fine like this ->

But on some old browser, my website looks ugly like this ->

Even failed connection like this ->

I use Hugo with Netlify and Let's Encrypt SSL. I don't know if this compatibility issue is from my code side, SSL side, server side, browser side, etc? Any idea how to fix this issue?

Regards