diff --git a/README.md b/README.md
index 5ca22854f..f36d52c07 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,9 @@ Feel free to stop by #pdfjs on irc.mozilla.org for questions or guidance.
 
 ### Online demo
 
+Please note that the "Modern browsers" version assumes native support for
+features such as e.g. `async`/`await`, `Promise`, and `ReadableStream`.
+
 + Modern browsers: https://mozilla.github.io/pdf.js/web/viewer.html
 
 + Older browsers: https://mozilla.github.io/pdf.js/es5/web/viewer.html