Although honestly it does look a little bit like a comment.

Maybe & are the way to go.

But...what about question mark? Question mark could be good. Or underscore.

  form {
    margin: 1rem 0.5rem;
    background: transparent;

    ? fieldset {
      border: none;
      padding: 0;
      background: white;
    }

    ? label {
      color: darkslategray;

      ? > input {
        border: thin solid;
      }

      ? input {
        border: none;
      }

      ? > button {
        font-size: larger;
      }
    }

    ? a {
      text-decoration: none;
    }

    ? :is(a, input, button, label) {
      color: inherit;
    }

    ?:invalid {
      color: red;
    }
  }

I think question mark's pretty good. It's the most semantic, visually for me. Maybe...I think.

What about underscore?

  form {
    margin: 1rem 0.5rem;
    background: transparent;

    _ fieldset {
      border: none;
      padding: 0;
      background: white;
    }

    _ label {
      color: darkslategray;

      _ > input {
        border: thin solid;
      }

      _ input {
        border: none;
      }

      _ > button {
        font-size: larger;
      }
    }

    _ a {
      text-decoration: none;
    }

    _ :is(a, input, button, label) {
      color: inherit;
    }

    _:invalid {
      color: red;
    }
  }

Looks weird to me. But some people might like it.