code: @Override
      public void execute() throws MojoExecutionException {
          if (skipCleanCSS) {
              getLog().debug("Skipping CSS minification");
              removeFromWatching();
              return;
          }
  
          cleancss = NPM.npm(this, CLEANCSS_NPM_NAME, cleanCssVersion);
          getLog().info("Clean CSS version: " + cleanCssVersion);
  
          // Check whether or not we have a custom configuration
          if (stylesheets == null) {
              getLog().info("No 'stylesheets' processing configuration, minifying all '.css' files individually");
              for (File file : getResources(getExtensions())) {
                  try {
                      process(file);
                  } catch (WatchingException e) {
                      throw new MojoExecutionException(e.getMessage(), e);
                  }
              }
          } else {
              process(stylesheets);
          }
      }
docs: Checks if the skipCleanCSS flag has been set if so, we stop watching css files. If not we
  continue by setting our Clean CSS NPM object and calling the minify method for all css
  files found.
  
  @throws MojoExecutionException when modification fails.