diff --git a/dom/observable/tentative/observable-every.any.js b/dom/observable/tentative/observable-every.any.js
new file mode 100644
index 00000000000000..74a344b8f78559
--- /dev/null
+++ b/dom/observable/tentative/observable-every.any.js
@@ -0,0 +1,250 @@
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.next("good");
+    subscriber.next("good");
+    subscriber.next("good");
+    subscriber.complete();
+  });
+
+  const result = await source.every((value) => value === "good");
+
+  assert_true(result, "Promise resolves with true if all values pass the predicate");
+}, "every(): Promise resolves to true if all values pass the predicate");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.next("good");
+    subscriber.next("good");
+    subscriber.next("bad");
+    subscriber.complete();
+  });
+
+  const result = await source.every((value) => value === "good");
+
+  assert_false(result, "Promise resolves with false if any value fails the predicate");
+}, "every(): Promise resolves to false if any value fails the predicate");
+
+promise_test(async () => {
+  let tornDown = false;
+  let subscriberActiveAfterFailingPredicate = true;
+  const source = new Observable(subscriber => {
+    subscriber.addTeardown(() => tornDown = true);
+    subscriber.next("good");
+    subscriber.next("good");
+    subscriber.next("bad");
+    subscriberActiveAfterFailingPredicate = subscriber.active;
+    subscriber.next("good");
+    subscriber.complete();
+  });
+
+  const result = await source.every((value) => value === "good");
+
+  assert_false(result, "Promise resolves with false if any value fails the predicate");
+  assert_false(subscriberActiveAfterFailingPredicate,
+      "Subscriber becomes inactive because every() unsubscribed");
+}, "every(): Abort the subscription to the source if the predicate does not pass");
+
+promise_test(async () => {
+  const logs = [];
+
+  const source = createTestSubject({
+    onSubscribe: () => logs.push("subscribed to source"),
+    onTeardown: () => logs.push("teardown"),
+  });
+
+  const resultPromise = source.every((value, index) => {
+    logs.push(`Predicate called with ${value}, ${index}`);
+    return true;
+  });
+
+  let promiseResolved = false;
+
+  resultPromise.then(() => promiseResolved = true);
+
+  assert_array_equals(logs, ["subscribed to source"],
+      "calling every() subscribes to the source immediately");
+
+  source.next("a");
+  assert_array_equals(logs, [
+    "subscribed to source",
+    "Predicate called with a, 0"
+  ], "Predicate called with the value and the index");
+
+  source.next("b");
+  assert_array_equals(logs, [
+    "subscribed to source",
+    "Predicate called with a, 0",
+    "Predicate called with b, 1",
+  ], "Predicate called with the value and the index");
+
+  // wait a tick, just to prove that you have to wait for complete to be called.
+  await Promise.resolve();
+
+  assert_false(promiseResolved,
+      "Promise should not resolve until after the source completes");
+
+  source.complete();
+  assert_array_equals(logs, [
+    "subscribed to source",
+    "Predicate called with a, 0",
+    "Predicate called with b, 1",
+    "teardown",
+  ], "Teardown function called immediately after the source completes");
+
+  const result = await resultPromise;
+
+  assert_true(result,
+      "Promise resolves with true if all values pass the predicate");
+}, "every(): Lifecycle checks when all values pass the predicate");
+
+promise_test(async () => {
+  const logs = [];
+
+  const source = createTestSubject({
+    onSubscribe: () => logs.push("subscribed to source"),
+    onTeardown: () => logs.push("teardown"),
+  });
+
+  const resultPromise = source.every((value, index) => {
+    logs.push(`Predicate called with ${value}, ${index}`);
+    return value === "good";
+  });
+
+  let promiseResolved = false;
+
+  resultPromise.then(() => promiseResolved = true);
+
+  assert_array_equals(logs, ["subscribed to source"],
+      "calling every() subscribes to the source immediately");
+
+  source.next("good");
+  source.next("good");
+  assert_array_equals(logs, [
+    "subscribed to source",
+    "Predicate called with good, 0",
+    "Predicate called with good, 1",
+  ], "Predicate called with the value and the index");
+
+  assert_false(promiseResolved, "Promise should not resolve until after the predicate fails");
+
+  source.next("bad");
+  assert_array_equals(logs, [
+    "subscribed to source",
+    "Predicate called with good, 0",
+    "Predicate called with good, 1",
+    "Predicate called with bad, 2",
+    "teardown",
+  ], "Predicate called with the value and the index, failing predicate immediately aborts subscription to source");
+
+  const result = await resultPromise;
+
+  assert_false(result, "Promise resolves with false if any value fails the predicate");
+}, "every(): Lifecycle checks when any value fails the predicate");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.complete();
+  });
+
+  const result = await source.every(() => true);
+
+  assert_true(result,
+      "Promise resolves with true if the observable completes without " +
+      "emitting a value");
+}, "every(): Resolves with true if the observable completes without " +
+   "emitting a value");
+
+promise_test(async t => {
+  const error = new Error("error from source");
+  const source = new Observable(subscriber => {
+    subscriber.error(error);
+  });
+
+  promise_rejects_exactly(t, error, source.every(() => true),
+      "Promise rejects with the error emitted from the source observable");
+}, "every(): Rejects with any error emitted from the source observable");
+
+promise_test(async t => {
+  const source = new Observable(subscriber => {
+    subscriber.next(1);
+    subscriber.next(2);
+    subscriber.next(3);
+    subscriber.complete();
+  });
+
+  const error = new Error("bad value");
+  const promise = source.every(value => {
+    if (value <= 2) return true;
+    throw error;
+  });
+
+  promise_rejects_exactly(t, error, promise, "Promise rejects with the " +
+      "error thrown from the predicate");
+}, "every(): Rejects with any error thrown from the predicate");
+
+promise_test(async () => {
+  const indices = [];
+
+  const source = new Observable(subscriber => {
+    subscriber.next("a");
+    subscriber.next("b");
+    subscriber.next("c");
+    subscriber.complete();
+  });
+
+  const value = await source.every((value, index) => {
+    indices.push(index);
+    return true;
+  });
+
+  assert_array_equals(indices, [0, 1, 2]);
+
+  assert_true(value,
+      "Promise resolves with true if all values pass the predicate");
+}, "every(): Index is passed into the predicate");
+
+promise_test(async t => {
+  const source = new Observable(subscriber => {});
+
+  const controller = new AbortController();
+  const promise = source.every(() => true, { signal: controller.signal });
+  controller.abort();
+
+  promise_rejects_dom(t, 'AbortError', promise, "Promise rejects with a " +
+      "DOMException if the source Observable is aborted");
+}, "every(): Rejects with a DOMException if the source Observable is aborted");
+
+function createTestSubject(options) {
+  const onTeardown = options?.onTeardown;
+
+  const subscribers = new Set();
+  const subject = new Observable(subscriber => {
+    options?.onSubscribe?.();
+    subscribers.add(subscriber);
+    subscriber.addTeardown(() => subscribers.delete(subscriber));
+    if (onTeardown) {
+      subscriber.addTeardown(onTeardown);
+    }
+  });
+
+  subject.next = (value) => {
+    for (const subscriber of Array.from(subscribers)) {
+      subscriber.next(value);
+    }
+  };
+  subject.error = (error) => {
+    for (const subscriber of Array.from(subscribers)) {
+      subscriber.error(error);
+    }
+  };
+  subject.complete = () => {
+    for (const subscriber of Array.from(subscribers)) {
+      subscriber.complete();
+    }
+  };
+  subject.subscriberCount = () => {
+    return subscribers.size;
+  };
+
+  return subject;
+}
diff --git a/dom/observable/tentative/observable-find.any.js b/dom/observable/tentative/observable-find.any.js
new file mode 100644
index 00000000000000..0e09060fc5ac19
--- /dev/null
+++ b/dom/observable/tentative/observable-find.any.js
@@ -0,0 +1,85 @@
+promise_test(async () => {
+  let inactiveAfterB = false;
+  const source = new Observable(subscriber => {
+    subscriber.next("a");
+    subscriber.next("b");
+    inactiveAfterB = !subscriber.active;
+    subscriber.next("c");
+    subscriber.complete();
+  });
+
+  const value = await source.find((value) => value === "b");
+
+  assert_equals(value, "b", "Promise resolves with the first value that passes the predicate");
+
+  assert_true(inactiveAfterB, "subscriber is inactive after the first value that passes the predicate");
+}, "find(): Promise resolves with the first value that passes the predicate");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.next("a");
+    subscriber.next("b");
+    subscriber.next("c");
+    subscriber.complete();
+  });
+
+  const value = await source.find(() => false);
+
+  assert_equals(value, undefined, "Promise resolves with undefined if no value passes the predicate");
+}, "find(): Promise resolves with undefined if no value passes the predicate");
+
+promise_test(async t => {
+  const error = new Error("error from source");
+  const source = new Observable(subscriber => {
+    subscriber.error(error);
+  });
+
+  promise_rejects_exactly(t, error, source.find(() => true), "Promise " +
+      "rejects with the error emitted from the source Observable");
+}, "find(): Promise rejects with the error emitted from the source Observable");
+
+promise_test(async t => {
+  const source = new Observable(subscriber => {
+    subscriber.next("ignored");
+  });
+
+  const error = new Error("thrown from predicate");
+  promise_rejects_exactly(t, error, source.find(() => {throw error}),
+      "Promise rejects with any error thrown from the predicate");
+}, "find(): Promise rejects with any error thrown from the predicate");
+
+promise_test(async () => {
+  let indices = [];
+
+  const source = new Observable(subscriber => {
+    subscriber.next("a");
+    subscriber.next("b");
+    subscriber.next("c");
+    subscriber.complete();
+  });
+
+  const value = await source.find((value, index) => {
+    indices.push(index);
+    return false;
+  });
+
+  assert_equals(value, undefined, "Promise resolves with undefined if no value passes the predicate");
+
+  assert_array_equals(indices, [0, 1, 2], "find(): Passes the index of the value to the predicate");
+}, "find(): Passes the index of the value to the predicate");
+
+promise_test(async t => {
+  const controller = new AbortController();
+  const source = new Observable(subscriber => {
+    subscriber.next("a");
+    subscriber.next("b");
+    subscriber.next("c");
+    subscriber.complete();
+  });
+
+  controller.abort();
+  const promise = source.find(() => true, { signal: controller.signal });
+
+  promise_rejects_dom(t, 'AbortError', promise, "Promise rejects with " +
+      "DOMException when the signal is aborted");
+}, "find(): Rejects with AbortError when the signal is aborted");
diff --git a/dom/observable/tentative/observable-some.any.js b/dom/observable/tentative/observable-some.any.js
new file mode 100644
index 00000000000000..b692610df329c6
--- /dev/null
+++ b/dom/observable/tentative/observable-some.any.js
@@ -0,0 +1,96 @@
+promise_test(async () => {
+  let inactiveAfterFirstGood = true;
+
+  const source = new Observable(subscriber => {
+    subscriber.next("good");
+    inactiveAfterFirstGood = !subscriber.active;
+    subscriber.next("good");
+    subscriber.next("good");
+    subscriber.complete();
+  });
+
+  const result = await source.some((value) => value === "good");
+
+  assert_true(result, "Promise resolves with true if any value passes the predicate");
+
+  assert_true(inactiveAfterFirstGood,
+      "subscriber is inactive after the first value that passes the " +
+      "predicate, because the source was unsubscribed from");
+}, "some(): subscriber is inactive after the first value that passes the predicate, because the source was unsubscribed from");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.next("bad");
+    subscriber.next("bad");
+    subscriber.next("bad");
+    subscriber.complete();
+  });
+
+  const result = await source.some((value) => value === "good");
+
+  assert_false(result, "some(): Promise resolves with false if no value passes the predicate");
+});
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.next("bad");
+    subscriber.next("bad");
+    subscriber.next("good");
+    subscriber.complete();
+  });
+
+  const result = await source.some((value) => value === "good");
+
+  assert_true(result, "some(): Promise resolves with true if any value passes the predicate");
+});
+
+promise_test(async t => {
+  const source = new Observable(subscriber => {
+    subscriber.next("not used");
+  });
+
+  const error = new Error("thrown from predicate");
+  promise_rejects_exactly(t, error, source.some(() => {throw error}),
+      "The returned promise rejects with an error if the predicate errors");
+}, "some(): The returned promise rejects with an error if the predicate errors");
+
+promise_test(async t => {
+  const error = new Error("error from source");
+  const source = new Observable(subscriber => {
+    subscriber.error(error);
+  });
+
+  promise_rejects_exactly(t, error, source.some(() => true),
+      "The returned promise rejects with an error if the source observable errors");
+}, "some(): The returned promise rejects with an error if the source observable errors");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.complete();
+  });
+
+  const result = await source.some(() => true);
+
+  assert_false(result,
+      "The returned promise resolves as false if the source observable " +
+      "completes without emitting a value");
+}, "some(): The returned promise resolves as false if the source observable " +
+   "completes without emitting a value");
+
+promise_test(async t => {
+  let teardownCalled = false;
+  const source = new Observable(subscriber => {
+    subscriber.addTeardown(() => {
+      teardownCalled = true;
+    });
+  });
+
+  const controller = new AbortController();
+  const promise = source.some(() => true, { signal: controller.signal });
+
+  controller.abort();
+
+  promise_rejects_dom(t, 'AbortError', promise);
+  assert_true(teardownCalled,
+      "The teardown function is called when the signal is aborted");
+}, "some(): The return promise rejects with a DOMException if the signal is aborted");