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+# RFC 43: Split 'assert\_precondition' into optional and non-optional variants
+
+## Summary
+
+Split the `assert_precondition` function into `assert_implements_optional` and
+`assert_implements`. The `assert_implements_optional` function will behave as
+`assert_precondition` does today; a failed assert will record a
+'PRECONDITION\_FAILED' status. The `assert_implements` function will be
+synactical sugar for `assert_true`.
+
+Reuse the 'PRECONDITION\_FAILED' status rather than renaming it due to the
+technical difficulties of getting mozlog updated.
+
+## Details
+
+### Background 
+
+The `assert_precondition` function and associated status
+('PRECONDITION\_FAILED') were added in [RFC #16](assert_precondition.md).
+However there has been confusion over whether the function is meant for spec
+features that are explicitly marked OPTIONAL (i.e. its usage in TODO), or
+whether it is meant as an early-out when a particular spec or feature is not
+implemented by a browser (i.e. its usage in TODO).
+
+The difference matters here due to the 'PRECONDITION\_FAILED' status, which has
+been interpreted by some users as a 'non-failing' status - which only makes
+sense if the failure is for truly OPTIONAL behavior.
+
+### Proposal
+
+Rename the `assert_precondition` function to `assert_implements_optional`, with
+the same 'PRECONDITION\_FAILED' status produced, and add a new function`
+assert_implements` that is essentially syntactical sugar for `assert_true`.
+That is:
+
+> __assert_implements(condition, description)__
+>
+> Concludes the test with a `FAIL` status if `condition` is not truthy.
+> Used to avoid running unnecessary code or subtests for a feature that is not
+> implemented by the running browser.
+
+> __assert_implements_optional(condition, description)__
+>
+> Concludes the test with a `PRECONDITION_FAILED` status if `condition` is not truthy.
+> Used for skipping tests or subtests for OPTIONAL features that that would otherwise
+> fail due to lack of support for the feature.
+
+The 'PRECONDITION\_FAILED' status is not being renamed, as changing the logging
+infrastructure (mozlog, etc) is logistically difficult.
+
+## Risks
+
+* As with any change in testharness.js, downstream embedders may have to spend
+  effort to adapt to the deprecation and rename. That said, there are currently
+  zero uses of `assert_precondition` in downstream Chromium tests.
+* As it is not being renamed, the 'PRECONDITION\_FAILED' status may confuse
+  consumers of WPT results.