function customMinifyCSS(originalCSS) {
  const result = cleanCSS.minify(originalCSS);

  if (result.warnings.length > 0 || result.errors.length > 0) {
    return originalCSS.trim();
  }

  return result.styles;
}
Minifies CSS, bailing on any warning or error parsing CSS. This is necessary because
lit-element allows partial invalid CSS, which trips up the minifier.

@param {string} originalCSS
@returns {string}