narrow browser support in postcss

build/postcss.config.js

@@ -11,16 +11,8 @@ module.exports = (ctx) => ({
         // 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/
+        'Chrome >= 45', // Exact version number here is kinda arbitrary
+        'Firefox ESR',
         // 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.
@@ -29,10 +21,9 @@ module.exports = (ctx) => ({
         'Explorer >= 10',
         // Out of leniency, we prefix these 1 version further back than the official policy.
         'iOS >= 8',
-        'Safari >= 8',
+        'Safari >= 9',
         // 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',
+        'Android >= 4.4',
         'Opera >= 12'
       ]
     }