Meta: appreciate web-platform-tests for normative changes

Part of #1849.

CONTRIBUTING.md

@@ -18,6 +18,10 @@ Please add your name to the Acknowledgements section (search for `<!-- ACKS`) in
 
 To preview your changes locally, follow the instructions in the [html-build repository](https://github.com/whatwg/html-build).
 
+#### Tests
+
+For normative changes, a corresponding [web-platform-tests](https://github.com/w3c/web-platform-tests) PR is highly appreciated. The author and reviewer can be different than for the spec change. If testing is not practical, please explain why and if appropriate file an issue to follow up later.
+
 #### Formatting
 
 Use a column width of 100 characters and add newlines where whitespace is used. (Emacs, set `fill-column` to `100`; in Vim, set `textwidth` to `100`; and in Sublime, set `wrap_width` to `100`.)