Bootstrap/grunt/postcss.js

module.exports = {
  use: [
    'postcss-flexbugs-fixes',
    'autoprefixer'
  ],
  map: {
    inline: false,
    annotation: true,
    sourcesContent: true
  },
  autoprefixer: {
    browsers: [
      //
      // Official browser support policy:
      // https://v4-alpha.getbootstrap.com/getting-started/browsers-devices/#supported-browsers
      //
      'Chrome >= 35', // Exact version number here is kinda arbitrary
      // Rather than using Autoprefixer's native "Firefox ESR" version specifier string,
      // we deliberately hardcode the number. This is to avoid unwittingly severely breaking the previous ESR in the event that:
      // (a) we happen to ship a new Bootstrap release soon after the release of a new ESR,
      //     such that folks haven't yet had a reasonable amount of time to upgrade; and
      // (b) the new ESR has unprefixed CSS properties/values whose absence would severely break webpages
      //     (e.g. `box-sizing`, as opposed to `background: linear-gradient(...)`).
      //     Since they've been unprefixed, Autoprefixer will stop prefixing them,
      //     thus causing them to not work in the previous ESR (where the prefixes were required).
      'Firefox >= 38', // Current Firefox Extended Support Release (ESR); https://www.mozilla.org/en-US/firefox/organizations/faq/
      // Note: Edge versions in Autoprefixer & Can I Use refer to the EdgeHTML rendering engine version,
      // NOT the Edge app version shown in Edge's "About" screen.
      // For example, at the time of writing, Edge 20 on an up-to-date system uses EdgeHTML 12.
      // See also https://github.com/Fyrd/caniuse/issues/1928
      'Edge >= 12',
      'Explorer >= 9',
      // Out of leniency, we prefix these 1 version further back than the official policy.
      'iOS >= 8',
      'Safari >= 8',
      // The following remain NOT officially supported, but we're lenient and include their prefixes to avoid severely breaking in them.
      'Android 2.3',
      'Android >= 4',
      'Opera >= 12'
    ]
  }
}