Prevent cssMin from removing svg styles

When running the grunt.js task cssMin https://github.com/gruntjs/grunt-contrib-cssmin

Svg css properties are being removed.

For example:

.testClass {
   r: 4;
   width: 3px;
   margin-left: 18px !important;
}

Gets converted to

.testClass {
   width: 3px;
   margin-left: 18px !important;
}

How can I prevent this from happening?

grunt-contrib-cssmin uses clean-css internally, as stated in options, and any native (clean-css) options "are passed to clean-css".

restructureRules: false, // should disable all removals
skipProperties: []       // specify individual rules to skip

cssmin: {
  options: {
    skipProperties: ['r']
  }
}

Starting with clean-css 4.2.0 a "comment" block method for skipping fragments entirely will be available:

/* clean-css ignore:start */
.testClass {
   r: 4;
   width: 3px;
   margin-left: 18px !important;
}
/* clean-css ignore:end */

Note 4.2 has not been released yet.

After a bit of testing, none of the above seems to actually work, although they "should", according to documentation.
The only alternative I have is replacing grunt-contrib-cssmin with grunt-postcss cssnano (which is what I use with grunt for minification):

npm install grunt-postcss cssnano

grunt.initConfig({
  postcss: {
    processors: [
      require('cssnano')({
        // options here...
      })
    ]
  },
});

grunt.loadNpmTasks('grunt-postcss');
grunt.registerTask('postcss', ["postcss"]);

In practice, I use more postcss addons.
Here's a practical example with autoprefixer, pixrem, postcss-flexbox and cssnano:

module.exports = function(grunt) {
    grunt.initConfig({
        postcss: {
            options: {
                processors: [
                    require('pixrem'),
                    require('autoprefixer')({browsers: ['> 0%']}),
                    require('postcss-flexboxfixer'),
                    require('cssnano')({
                      autoprefixer:false,
                      zindex: false
                    })
                ]
            },
            jack: {
                files: [{
                    expand:true,
                    flatten:true,
                    cwd: 'assets/',
                    src: ['scss/*.css', '!**/variables.css'],
                    dest:'assets/css/'
                }]
            }
        },
        watch: {
            styles: {
                files: [
                    'assets/scss/*.css'
                ],
                tasks:['postcss:jack']
            }
        }
    });
    grunt.loadNpmTasks('grunt-postcss');
    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.registerTask('default', ["watch:styles"]);
    grunt.registerTask('jack', ["postcss:jack"]);
};

I purposefully registered a task that only runs the postcss plugin:

grunt jack

Don't forget you need to install each addon for usage with postcss. For the above, you'd need:

npm install grunt-postcss cssnano pixrem autoprefixer postcss-flexboxfixer

... and, most likely, you'll want to change files to match whatever you have.

This time around, I tested. The r property makes it into the minified file:

.testClass{r:20;width:3px;margin-left:18px!important}