You kinda can:

  selector {
    //property: value;
  }

This sets the property named "//property" to "value". Close enough. (Expressed otherwise: it’s approximately a comment until the next semicolon, barring quoted strings and parentheses.)

  //selector {
    property: value;
  }

This is an invalid selector, and so the entire rule is ignored. (Expressed otherwise: it’s approximately a comment until the next opening curly brace’s matching closing curly brace.)