diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 6ef0494edc..7a8a44f211 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -33,6 +33,7 @@ We only accept issues that are bug reports or feature requests. Bugs must be iso
 - Double quotes only, never single quotes
 - Always use proper indentation
 - Use tags and elements appropriate for an HTML5 doctype (e.g., self-closing tags)
+- Use CDNs and HTTPS for third-party JS when possible. We don't use protocol-relative URLs in this case because they break when viewing the page locally via `file://`.
 
 ### CSS