code: function supportsCssVariables(forceRefresh = false) {
    if (typeof supportsCssVariables_ === 'boolean' && !forceRefresh) {
      return supportsCssVariables_;
    }
  
    const supportsFunctionPresent =
      win.CSS && typeof win.CSS.supports === 'function';
    if (!supportsFunctionPresent) {
      return;
    }
  
    const explicitlySupportsCssVars = win.CSS.supports('--css-vars', 'yes');
    /*
     * See: https://bugs.webkit.org/show_bug.cgi?id=154669
     * See: README section on Safari
     */
    const weAreFeatureDetectingSafari10plus =
      win.CSS.supports('(--css-vars: yes)') &&
      win.CSS.supports('color', '#00000000');
  
    if (explicitlySupportsCssVars || weAreFeatureDetectingSafari10plus) {
      supportsCssVariables_ = !detectEdgePseudoVarBug();
    } else {
      supportsCssVariables_ = false;
    }
    return supportsCssVariables_;
  }
docs: 判断是否支持 css 变量
  https://caniuse.com/#feat=css-variables
  当前支持的浏览器有：
  Edge 15+ (15对于伪类有 bug)、Firefox 31+ 、Chrome 49+ 、Safari 9.1+、Opera 36+
  IOS Safari 9.3+ 、(Android 5+: Chromium 62+ Opera 37+ Chrome 62+ FireFox 57+ )
  @param {boolean=} forceRefresh
  @return {boolean|undefined}