code: function customPropertiesSupported () {
    if (typeof cachedReturnVal !== 'undefined') return cachedReturnVal
    return cachedReturnVal = (
      canUseDOM &&
      !(Browser.msedge) && // polyfill edge until improved css variable support
      window.CSS && window.CSS.supports && window.CSS.supports('color', 'var(--primary)')
    )
  }
docs: ---
  category: utilities/DOM
  ---
  check if CSS custom properties (CSS variables) are supported
  @returns {Boolean} true if the DOM is available and CSS variables are supported