this plugin seems doesn't work with react

[LastStranger]

I've tried many ways, but none of them works.

module.exports = ({env}) => ({
  plugins: {
      'postcss-uncss':{
        "html": ['http://localhost:3456'],
        //   html: ['src/**/*.html'],
        //   html: ['src/index.html'],
      },
 }
});

the rest of two don't show any error ,but my output css file is empty.