function checkElide() {
      elide = blockStack.length !== 0
          && blockStack[blockStack.length-1] !== null;
    }
@param {string} cssText a string containing a CSS stylesheet.
@param {string} suffix a suffix that is added to all IDs and which is
   used as a CLASS names so that the returned selectors will only match
   nodes under one with suffix as a class name.
   If suffix is {@code "sfx"}, the selector
   {@code ["a", "#foo", " ", "b", ".bar"]} will be namespaced to
   {@code [".sfx", " ", "a", "#foo-sfx", " ", "b", ".bar"]}.
@param {function(string, string)} opt_naiveUriRewriter maps URLs of media
   (images, sounds) that appear as CSS property values to sanitized
   URLs or null if the URL should not be allowed as an external media
   file in sanitized CSS.