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+# Origin-keyed agent clusters tests
+
+These are tests for the [origin-keyed agent clusters](https://html.spec.whatwg.org/multipage/origin.html#origin-keyed-agent-clusters)
+feature.
+
+## Test filenames
+
+The tests in `2-iframes` follow the file naming pattern
+
+```
+parent-[yes|no]-child1-[yes|no]-[designator]-child2-[yes|no]-[designator]
+```
+
+Here:
+
+* `yes` or `no` refers to whether the `Origin-Agent-Cluster` header is set or
+  unset.
+* `designator` explains how the child differs from the parent: e.g. by being a
+  subdomain, or having a different port, or both. There's also `same` if it's
+  same-origin.
+
+Other directories have variations on this, e.g. `1-iframe/` does the same thing
+but for a single `child` instead of `child1` and `child2`, and `navigation/`
+uses `1` and `2` to represent the two different locations the single iframe will
+be navigated to.
+
+## Coverage
+
+Header parsing is covered by a few tests in the `1-iframe/` subdirectory, and
+not duplicated to all other scenarios.