function isOptimalCssBuild(cssBuild = '') {
  // CSS custom property shim always requires work
  if (cssBuild === '' || !nativeCssVariables) {
    return false;
  }
  return nativeShadow ? cssBuild === 'shadow' : cssBuild === 'shady';
}
Check if the css build status is optimal, and do no unneeded work.

@param {string=} cssBuild CSS build status
@return {boolean} css build is optimal or not