Update and slighly expand WPT docs

docs/testing/web_platform_tests.md

 # Web Platform Tests
 
-The Web Platform Tests project provides a large number of conformance tests for
-different aspects of the Web Platform. Currently the tests are hosted on GitHub.
-There are two main repositories, one for the CSS Working Group (aka csswg-test),
-and one for pretty much everything else (aka web-platform-tests).
-
-There is a plan to merge csswg-test into web-platform-tests, so later, there
-will be only one repository.
+Interoperability between browsers is
+[critical](https://www.chromium.org/blink/platform-predictability) to
+Chromium's mission of improving the web. We believe that leveraging and
+contributing to a shared test suite is one of the most important tools in
+achieving interoperability between browsers.  The [web-platform-tests
+repository](https://github.com/w3c/web-platform-tests) is the primary shared
+test suite where all browser engines are collaborating.  There's also a
+[csswg-test repository](https://github.com/w3c/csswg-test) for CSS tests, but
+that will [soon be merged into
+web-platform-tests](https://github.com/w3c/csswg-test/issues/1102).
 
 [TOC]
 
 ## Import
 
 Chromium has mirrors
 ([web-platform-tests](https://chromium.googlesource.com/external/w3c/web-platform-tests/),
 [csswg-test](https://chromium.googlesource.com/external/w3c/csswg-test/)) of the
 GitHub repos, and periodically imports a subset of the tests so that they are
 run as part of the regular Blink layout test testing process.
@@ skipping 43 matching lines @@
 it, and on the next auto-import, the new tests should be imported. If you want
 to import immediately, you can also run `wpt-import --allow-local-commits`.
 
 ## Contributing tests back to the Web Platform Tests project.
 
 If you need to make changes to Web Platform Tests, just commit your changes
 directly to
 [LayoutTests/external/wpt](../../third_party/WebKit/LayoutTests/external/wpt)
 and the changes will be automatically upstreamed within 24 hours.
 
+Note that tests in Web Platform Tests are expected to match behavior defined by
+the relevant WHATWG or W3C specification, not simply Blink's behavior.  If in
+doubt, please request code review from someone with expertise in the relevant
+specification text.
+
 Note: if you're adding a new test in `external/wpt`, you'll need to re-generate
 MANIFEST.json manually until [CL 2644783003](https://crrev.com/2644783003) is
 landed. The command to do so is:
 
 ```bash
 Tools/Scripts/webkitpy/thirdparty/wpt/wpt/manifest --work \
     --tests-root=LayoutTests/external/wpt
 ```
 
 ### Will the exported commits be linked to my GitHub profile?
 
 The email you commit with in Chromium will be the author of the commit on
 GitHub. You can [add it as a secondary address on your GitHub
 account](https://help.github.com/articles/adding-an-email-address-to-your-github-account/)
 to link your exported commits to your GitHub profile.
 
 ### What if there are conflicts?
 
 This cannot be avoided entirely as the two repositories are independent, but
 should be rare with frequent imports and exports. When it does happen, manual
 intervention will be needed and in non-trivial cases you may be asked to help
 resolve the conflict.
 
 ### Direct pull requests
 
 It's still possible to make direct pull requests to web-platform-tests. The
 processes for getting new tests committed the W3C repos are at
 http://testthewebforward.org/docs/. Some specifics are at
 http://testthewebforward.org/docs/github-101.html.