feat(v2): postcss should only use stage 3 features instead of stage 2 (#1872)

CHANGELOG-2.x.md

@@ -4,6 +4,7 @@
 
 - Add sticky footer.
 - Remove empty doc sidebar container
+- PostCSS preset env now only polyfills Stage 3 features (previously it was stage 2) like Create React App. Stage 2 CSS is considered relatively unstable and subject to change while Stage 3 features will likely become a standard.
 
 ## 2.0.0-alpha.30
 

packages/docusaurus/src/webpack/utils.ts

@@ -54,6 +54,7 @@ export function getStyleLoaders(
             autoprefixer: {
               flexbox: 'no-2009',
             },
+            stage: 3,
           }),
         ],
       },