Add giant comment about why we're runnign CSS through PostCSS

webpack.config.js

@@ -159,6 +159,20 @@ module.exports = (env, argv) => ({
                                 // Note that we use significantly fewer plugins on the plain
                                 // CSS parser. If we start to parse plain CSS, we end with all
                                 // kinds of nasty problems (like stylesheets not loading).
+                                //
+                                // You might have noticed that we're also sending regular CSS
+                                // through PostCSS. This looks weird, and in fact is probably
+                                // not what you'd expect, however in order for our CSS build
+                                // to work nicely we have to do this. Because down the line
+                                // our SCSS stylesheets reference plain CSS we have to load
+                                // the plain CSS through PostCSS so it can find it safely. This
+                                // also acts like a babel-for-css by transpiling our (S)CSS
+                                // down/up to the right browser support (prefixes, etc).
+                                // Further, if we don't do this then PostCSS assumes that our
+                                // plain CSS is SCSS and it really doesn't like that, even
+                                // though plain CSS should be compatible. The chunking options
+                                // at the top of this webpack config help group the SCSS and
+                                // plain CSS together for the bundler.
 
                                 require("postcss-simple-vars")(),
                                 require("postcss-strip-inline-comments")(),