Meta: appreciate web-platform-tests for normative changes

Part of #1849.

CONTRIBUTING.md

@@ -14,6 +14,10 @@ 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 from the HTML Standard PR. If testing is not practical, please explain why and if appropriate [file an issue](https://github.com/w3c/web-platform-tests/issues/new) 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`.)

TEAM.md

@@ -6,7 +6,7 @@ This document contains info used by the team maintaining the standard. Mostly bo
 
 Each change needs to result in a single commit on the master branch, with no merge commits. The green squash and merge button is OK to use, but be sure to tidy up the commit message per [guidelines for writing good commit messages](https://github.com/erlang/otp/wiki/Writing-good-commit-messages).
 
-For optimal merges, the following instructions may be helpful:
+For normative changes, ask for a [web-platform-tests](https://github.com/w3c/web-platform-tests) PR if testing is practical and not overly burdensome. If a follow up issue is filed, add the `html` label.
 
 ### Fetching and reviewing pull requests from forks