Add browserslist config for autoprefixer (#3053)

We will reduce the weight of the style sheet by specifying the target web browser of autoprefixer.

.postcssrc.yml

 plugins:
   postcss-smart-import: {}
   precss: {}
-  autoprefixer: {}
+  autoprefixer:
+    browsers:
+      - last 2 versions
+      - IE >= 11
+      - iOS >= 9