Bug 1483498 [wpt PR 12493] - Document rough consensus on reverting changes, a=testonly

Automatic update from web-platform-testsDocument rough consensus on reverting changes (#12493)

Fixes https://github.com/web-platform-tests/wpt/issues/9953.
--

wpt-commits: 7abda4e7227b65fba2452980d106ed5f11d51b2b
wpt-pr: 12493

testing/web-platform/meta/MANIFEST.json

@@ -274628,6 +274628,11 @@
      {}
     ]
    ],
+   "docs/_appendix/reverting.md": [
+    [
+     {}
+    ]
+   ],
    "docs/_appendix/test-templates.md": [
     [
      {}
@@ -580393,6 +580398,10 @@
    "12cfdfa13ade604dec791174ffcd2e3732c7a185",
    "support"
   ],
+  "docs/_appendix/reverting.md": [
+   "1f549b3a341f2a78a16be835ae43e2d8da5b7137",
+   "support"
+  ],
   "docs/_appendix/test-templates.md": [
    "39b599569876967d6de158c6497ecff59dd7bbef",
    "support"

testing/web-platform/tests/docs/_appendix/reverting.md

@@ -0,0 +1,31 @@
+---
+layout: page
+title: Reverting Changes
+order: 3
+---
+Testing is imperfect and from time to time changes are merged into master which
+break things for users of web-platform-tests. Such breakage can include:
+
+  * Failures in Travis or Taskcluster runs for this repository, either on the
+    master branch or on pull requests following the breaking change.
+
+  * Breakage in browser engine repositories which import and run
+    web-platform-tests, such as Chromium, Edge, Gecko, Servo and WebKit.
+
+  * Breakage in results collections systems for results dashboards, such as
+    [wpt.fyi](https://wpt.fyi).
+
+  * Breakage in supplemental tooling used by working groups, such as the
+    [CSS build system][].
+
+When such breakage happens, if the maintainers of the affected systems request
+it, pull requests to revert the original change should normally be approved and
+merged as soon as possible. (When the original change itself was fixing a
+serious problem, it's a judgement call, but prefer the fastest path to a stable
+state acceptable to everyone.)
+
+Once a revert has happened, the maintainers of the affected systems are
+expected to work with the original patch author to resolve the problem so that
+the change can be relanded. A reasonable timeframe to do so is within one week.
+
+[CSS build system]: https://github.com/web-platform-tests/wpt/tree/master/css/tools