code: function cssToHead (css, id, once) {
    if (once === undefined) once = true;
    if (once) {
      if (___inserted.indexOf(id) === -1) {
        injectCss(css, id);
        ___inserted.push(id);
      }
    } else {
      injectCss(css, id);
      ___inserted.push(id);
    }
  }
docs: Inject the css string into the <head> tag within a <style> tag. If once is set to true,
  the css matching this ID will only be injected once
  
  @param {string} css the css that will be injected into the style
  @param {string} id the ID value of the style tag, should be unique
  @param {?boolean} once if the css matching the ID can only be injected once (=true)