code: function processFallbackRules(root) {
    // an output for each execution mode
  
    // Reuse existing output in memory -- fixes issues with build "forgetting" about CSS vars previously working in IE 11 when compiling more than one thing at a time.
    if (!output) {
      output = {
        [ExecutionMode.CSS_COLOR]: [],
      };
    }
  
    // define which modes need to be processed
    const execModes = [ExecutionMode.CSS_COLOR];
  
    walkFallbackAtRules(root, execModes, output);
    walkFallbackRules(root, execModes, output);
  
    writeFallbackCSS(output);
  }
docs: Walk through all rules, and generate a CSS fallback for legacy browsers.
  One file is created for full compatibility:
  1. A CSS file, contains all the rules with the original color representation.
  @param root