PostCSS-only gulp setup

0_package.json

"dependencies": {
    "autoprefixer-core": "^5.1.8",
    "csswring": "^3.0.2",
    "gulp": "~3.8.10",
    "postcss": "^4.0.4",
    "postcss-custom-properties": "^3.0.1",
    "postcss-import": "^5.0.2",
    "postcss-mixins": "^0.1.1",
    "postcss-nested": "^0.2.1",
  },

1_gulpfile.js

var gulp = require('gulp');
var postcss = require('gulp-postcss');

gulp.task('css', function() {
    var processors = [
        require("postcss-import")(),
        require("autoprefixer-core"),
        require("postcss-custom-properties")(),
        require("postcss-calc")(),
        require("postcss-mixins"),
        require("postcss-nested"),
        require("csswring").postcss
    ];
    return gulp.src(['./src/styles/styles.css'])
        .pipe(postcss(processors))
        .pipe(gulp.dest(paths.dest.css));
});

2_mixins.css

@define-mixin nomargin {
    margin: calc(100000 * 0);
}

3_styles.css

:root{
    --main-color: #00AEFF
}

@import "/mixin.css"

*, *:before, *:after {
    box-sizing: border-box;
}

body {
    @mixin nomargin;
}

.model {
    display: flex;
    flex-direction: column;
    @mixin nomargin;
    color: var(--main-color);
    font-family: 'Noto Serif', serif;
    header {
        display: flex;
        justify-content: space-around;
        h3 {
            flex:1;
        }
    }
}

4_prettifiedResultCss.css

body {
    margin: 0
}

.model {
    display: -webkit-box;
    display: -webkit-flex;
    display: -ms-flexbox;
    display: flex;
    -webkit-box-orient: vertical;
    -webkit-box-direction: normal;
    -webkit-flex-direction: column;
    -ms-flex-direction: column;
    flex-direction: column;
    margin: 0;
    color: #00aeff
}

.model header {
    display: -webkit-box;
    display: -webkit-flex;
    display: -ms-flexbox;
    display: flex;
    -webkit-justify-content: space-around;
    -ms-flex-pack: distribute;
    justify-content: space-around
}

.model header h3 {
    -webkit-box-flex: 1;
    -webkit-flex: 1;
    -ms-flex: 1;
    flex: 1
}

5_realResultStyles.css

body{margin:0}.model{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-orient:vertical;-webkit-box-direction:normal;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;margin:0;color:#00aeff;font-family:Noto Serif,serif}.model header{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-justify-content:space-around;-ms-flex-pack:distribute;justify-content:space-around}.model header h3{-webkit-box-flex:1;-webkit-flex:1;-ms-flex:1;flex:1}