Update web-platform-tests to revision 20a217cb8488e4339e0c9610aba99e2654b676c3

tests/wpt/web-platform-tests/docs/writing-tests/testharness-api.md

@@ -313,40 +313,44 @@ NOTE: All asserts must be located in a `test()` or a step of an
 these places won't be detected correctly by the harness and may cause
 unexpected exceptions that will lead to an error in the harness.
 
-## Preconditions ##
+## Optional Features ##
 
-When a test would be invalid unless certain conditions are met, but yet
-doesn't explicitly depend on those preconditions, `assert_precondition` can be
-used. For example:
+If a test depends on a specification or specification feature that is OPTIONAL
+(in the [RFC 2119 sense](https://tools.ietf.org/html/rfc2119)),
+`assert_implements_optional` can be used to indicate that failing the test does
+not mean violating a web standard. For example:
 
 ```js
 async_test((t) => {
   const video = document.createElement("video");
-  assert_precondition(video.canPlayType("video/webm"));
+  assert_implements_optional(video.canPlayType("video/webm"));
   video.src = "multitrack.webm";
   // test something specific to multiple audio tracks in a WebM container
   t.done();
 }, "WebM with multiple audio tracks");
 ```
 
-A failing `assert_precondition` call is reported as a status of
-`PRECONDITION_FAILED` for the subtest.
+A failing `assert_implements_optional` call is reported as a status of
+`PRECONDITION_FAILED` for the subtest. This unusual status code is a legacy
+leftover from the deprecated `assert_precondition`; see the [RFC that renamed
+it](https://github.com/web-platform-tests/rfcs/pull/48).
 
-`assert_precondition` can also be used during test setup. For example:
+`assert_implements_optional` can also be used during test setup. For example:
 
 ```js
 setup(() => {
-  assert_precondition("onfoo" in document.body, "'foo' event supported");
+  assert_implements_optional("optionalfeature" in document.body,
+                             "'optionalfeature' event supported");
 });
-async_test(() => { /* test #1 waiting for "foo" event */ });
-async_test(() => { /* test #2 waiting for "foo" event */ });
+async_test(() => { /* test #1 waiting for "optionalfeature" event */ });
+async_test(() => { /* test #2 waiting for "optionalfeature" event */ });
 ```
 
-A failing `assert_precondition` during setup is reported as a status of
+A failing `assert_implements_optional` during setup is reported as a status of
 `PRECONDITION_FAILED` for the test, and the subtests will not run.
 
-See also the `.optional` [file name convention](file-names.md), which is
-appropriate when the precondition is explicitly optional behavior.
+See also the `.optional` [file name convention](file-names.md), which may be
+preferable if the entire test is optional.
 
 ## Cleanup ##
 
@@ -772,10 +776,6 @@ workers and want to ensure they run in series:
 
 ## List of Assertions ##
 
-### `assert_precondition(condition, description)`
-asserts that `condition` is truthy.
-See [preconditions](#preconditions) for usage.
-
 ### `assert_true(actual, description)`
 asserts that `actual` is strictly true
 
@@ -885,6 +885,13 @@ that the exception should have as its .constructor.  For example,
 
 `func` - a function that should throw
 
+### `assert_implements(condition, description)`
+asserts that a feature is supported, by checking if `condition` is truthy.
+
+### `assert_implements_optional(condition, description)`
+asserts that an optional feature is supported, by checking if `condition` is truthy.
+See [Optional Features](#optional-features) for usage.
+
 ### `assert_unreached(description)`
 asserts if called. Used to ensure that some codepath is *not* taken e.g.
 an event does not fire.
@@ -897,6 +904,9 @@ asserts that one `assert_func(actual, expected_array_N, extra_arg1, ..., extra_a
   allows multiple behaviours. Test authors should not use this method simply to hide
   UA bugs.
 
+### **DEPRECATED** `assert_precondition(condition, description)`
+Use `assert_implements` or `assert_implements_optional` instead.
+
 ## Utility functions ##
 
 ### **DEPRECATED** `on_event(object, event, callback)`