cssnano removing prefixes

[vlrprbttst]

i'm using "cssnano": "^3.4.0", with grunt.
here's my config

        postcss: {
            dev: {
                options: {
                    map: true,
                    processors: [
                        require('autoprefixer')({
                            browsers: 'last 2 version, IE 9'
                        }),

                    ]
                },
                src: '_dev/css/main.css'
            },
            build: {
                options: {
                    map: false,
                    processors: [
                        require('cssnano')()
                    ]
                },
                src: '_site/css/main.css'
            }
        }, //postcss

this

  -webkit-transform: rotate(129deg);
      -ms-transform: rotate(129deg);
          transform: rotate(129deg);

becomes this after the cssnano treatment:

    -webkit-transform: rotate(129deg);
    transform: rotate(129deg)

I'm supposed to keep the -ms-, why is it removed?

[ben-eb]

You need to pass the browsers option with cssnano's autoprefixer.

        postcss: {
            dev: {
                options: {
                    map: true,
                    processors: [
                        require('autoprefixer')({
                            browsers: 'last 2 version, IE 9'
                        }),

                    ]
                },
                src: '_dev/css/main.css'
            },
            build: {
                options: {
                    map: false,
                    processors: [
                        require('cssnano')({autoprefixer: {browsers: 'last 2 version, IE 9'}})
                    ]
                },
                src: '_site/css/main.css'
            }
        }, //postcss