grunt-legacssy demo

Gruntfile.js

grunt.loadNpmTasks('grunt-legacssy');

grunt.initConfig({
  legacssy: {
    files: {
      'css/style-legacy.css': 'css/style.css',
    },
  },
})

index.html

<!--[if lte IE 8]>     <link rel="stylesheet" href="css/style-legacy.css"> <![endif]-->
<!--[if gt IE 8]><!--> <link rel="stylesheet" href="css/style.css"> <!--<![endif]-->

style-legacy.css

body {
  margin: 0;
  padding: 0;
}

h1 {
  font-weight: bold;
}

style.css

body {
  margin: 0;
  padding: 0;
}

@media only screen and (min-width: 768px) {
  h1 {
    font-weight: bold;
  }
}