Build system overhaul.

build/postcss.config.js

@@ -0,0 +1,41 @@
+module.exports = (ctx) => ({
+  map: ctx.file.dirname.startsWith('docs') ? false : {
+    inline: false,
+    annotation: true,
+    sourcesContent: true
+  },
+  plugins: {
+    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 >= 10',
+        // 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'
+      ]
+    },
+    'postcss-flexbugs-fixes': {}
+  }
+})