[myfonj]

These are (presumably) for things that even user styles are not supposed to be able to "break". Sifting through https://meiert.com/en/blog/user-agent-style-sheets/ shows quite reasonable things such as

    @media (scripting) {
      noscript {
        display: none !important;
      }
    }

    optgroup {
      float: none !important;
      position: static !important;
      white-space: nowrap !important;
      word-wrap: normal !important;
    }

in the Firefox UA styles (though I'd permit overriding the former and I'd expect shadow DOM to handle the latter), or

    input[type="password" i] {
        -webkit-text-security: disc !important;
    }

in the Chrome UA style.