Why is grunt-postcss autoprefixer not working?

Question

Grunt.js

Here is the code from Gruntfile.js.

postcss: {
       options: {
         processors: [
           require("autoprefixer")({browsers:
           [
             "last 1 version",
             "last 2 Chrome versions",
             "last 2 Firefox versions",
             "last 2 Opera versions",
             "last 2 Edge versions"
           ]})
         ]
       },
       style: {src: "css/*.css"}
     }

The task works, but I don't see any changes in the CSS file. What could be the problem and what are the solutions? Thanks in advance.