Merge branch '6.4' into 7.0

* 6.4:
  [AssetMapper] Clarifying `importmap_polyfill`
  [Frontend] Adding link to PostCSS

Author: javiereguiluz

frontend.rst

@@ -48,7 +48,7 @@ be executed by a browser.
 
 .. _ux-note-2:
 
-**[2]** There are some PostCSS plugins available to remove comments from CSS files.
+**[2]** There are some `PostCSS`_ plugins available to remove comments from CSS files.
 
 .. _frontend-asset-mapper:
 
@@ -109,9 +109,10 @@ Other Front-End Articles
 .. _`Webpack`: https://webpack.js.org/
 .. _`Node.js`: https://nodejs.org/
 .. _`Webpack Encore screencast series`: https://symfonycasts.com/screencast/webpack-encore
-.. _StimulusBundle Documentation: https://symfony.com/bundles/StimulusBundle/current/index.html
-.. _Stimulus/UX: https://symfony.com/bundles/StimulusBundle/current/index.html
-.. _Stimulus: https://stimulus.hotwired.dev/
-.. _Turbo: https://turbo.hotwired.dev/
-.. _Symfony UX: https://ux.symfony.com
-.. _API Platform: https://api-platform.com/
+.. _`StimulusBundle Documentation`: https://symfony.com/bundles/StimulusBundle/current/index.html
+.. _`Stimulus/UX`: https://symfony.com/bundles/StimulusBundle/current/index.html
+.. _`Stimulus`: https://stimulus.hotwired.dev/
+.. _`Turbo`: https://turbo.hotwired.dev/
+.. _`Symfony UX`: https://ux.symfony.com
+.. _`API Platform`: https://api-platform.com/
+.. _`PostCSS`: https://postcss.org/

frontend/asset_mapper.rst

@@ -930,15 +930,19 @@ This option is enabled by default.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Configure the polyfill for older browsers. By default, the `ES module shim`_ is loaded
-via a CDN. You can pass the key of an item in ``importmap.php`` or ``false`` to disable
-the polyfill loading.
+via a CDN (i.e. the default value for this setting is `es-module-shims`):
 
 .. code-block:: yaml
 
     framework:
         asset_mapper:
-            importmap_polyfill: false # disable the shim ...
-            # importmap_polyfill: 'my_import_map' # ... or pass an importmap name
+            # set this option to false to disable the shim entirely
+            # (your website/web app won't work in old browsers)
+            importmap_polyfill: false
+
+            # you can also use a custom polyfill by adding it to your importmap.php file
+            # and setting this option to the key of that file in the importmap.php file
+            # importmap_polyfill: 'custom_polyfill'
 
 .. tip::