diff --git a/packages/block-editor/src/components/iframe/index.js b/packages/block-editor/src/components/iframe/index.js
index f9ef74b8cd1e49..293742f55683c5 100644
--- a/packages/block-editor/src/components/iframe/index.js
+++ b/packages/block-editor/src/components/iframe/index.js
@@ -306,6 +306,9 @@ function Iframe( {
 		iframeDocument.documentElement.classList.add( 'is-zoomed-out' );
 
 		const maxWidth = 750;
+		// This scaling calculation has to happen within the JS because CSS calc() can
+		// only divide and multiply by a unitless value. I.e. calc( 100px / 2 ) is valid
+		// but calc( 100px / 2px ) is not.
 		iframeDocument.documentElement.style.setProperty(
 			'--wp-block-editor-iframe-zoom-out-scale',
 			scale === 'default'