DOC-3243: Bundled content CSS is now loaded into preview iframes (#4014)

* Docs: DOC-3243 - Add release note for bundled content CSS in preview iframes

* Update modules/ROOT/pages/8.4.0-release-notes.adoc

Co-authored-by: Sorita Heng <69398037+soritaheng@users.noreply.github.com>

---------

Co-authored-by: Sorita Heng <69398037+soritaheng@users.noreply.github.com>

Author: kemister85

modules/ROOT/pages/8.4.0-release-notes.adoc

@@ -170,6 +170,14 @@ Previously, right-to-left (RTL) directionality was not correctly set for Hebrew
 
 In {productname} {release-version}, this issue has been addressed by ensuring that the directionality is correctly set for Hebrew in the language pack.
 
+=== Bundled content CSS is now loaded into preview iframes
+// #TINY-13190
+
+Previously, when the editor content CSS was bundled, it was not loaded into preview iframes. This caused a visual mismatch between the editor content area and what users saw in the Preview dialog, Suggested Edits view and Revision History view.
+
+In {productname} {release-version}, the fix checks if the content CSS is available as a bundled resource and applies it to the Preview Iframe through style tags. Bundled content CSS is now correctly loaded in the Preview Iframe.
+
+
 [[security-fixes]]
 == Security fixes