diff --git a/platform/commonUI/README.md b/platform/commonUI/README.md
index f77df6aa2f..567a9e0999 100644
--- a/platform/commonUI/README.md
+++ b/platform/commonUI/README.md
@@ -1,3 +1,17 @@
-This directory contains bundles containing common user interface 
-elements of Open MCT Web; that is, the user interface for the application 
-as a whole (as opposed to for specific features) is implemented here. 
+This directory contains bundles containing common user interface
+elements of Open MCT Web; that is, the user interface for the application
+as a whole (as opposed to for specific features) is implemented here.
+
+# Extensions
+
+This bundles adds a `stylesheets` extension category used to inject CSS
+from bundles. These extensions are declaration-only (no scripted
+implementation is needed or used); a single property, `stylesheetUrl`,
+should be provided, with a path to the relevant CSS file (including
+extension) relative to the resources directory for that bundle.
+
+Links to these CSS files are appended to the head when the application
+is started. These are added in standard priority order (see documentation
+for the framework layer); the order of inclusion of style sheets can
+change the way they are handled/understood by the browser, so priority
+can be used to provide control over this order.
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