diff --git a/html/semantics/interactive-elements/the-popup-element/popup-anchor-nesting.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-anchor-nesting.tentative.html
index f39d301e2264c5..64b8fc694c73c5 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-anchor-nesting.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-anchor-nesting.tentative.html
@@ -2,7 +2,7 @@
 <meta charset="utf-8" />
 <title>Popup anchor nesting</title>
 <link rel="author" href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="/resources/testdriver.js"></script>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-element-basic.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-element-basic.tentative.html
index 9909b2f0bbfa3c..0b1d99099e586f 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-element-basic.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-element-basic.tentative.html
@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <link rel="author" href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-hidden-display.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-hidden-display.tentative.html
index 8a070b9d479a55..b313f06f664d35 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-hidden-display.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-hidden-display.tentative.html
@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <link rel=author href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <link rel=match href="popup-hidden-display-ref.tentative.html">
 
 No popup should be displayed here.<p>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen-display-ref.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen-display-ref.tentative.html
new file mode 100644
index 00000000000000..db1a3f2b044d17
--- /dev/null
+++ b/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen-display-ref.tentative.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel=author href="mailto:masonf@chromium.org">
+
+<div>This is a popup, which should be open upon load</div>
+
+<style>
+  div {
+    /* Per spec: */
+    display: block;
+    position: fixed;
+    top: 0;
+    left: 0;
+    /* Per settings in test file: */
+    width: fit-content;
+    height: fit-content;
+    border: 1px solid;
+    padding: 1em;
+    background: -internal-light-dark(white, black);
+    color: -internal-light-dark(black, white);
+  }
+</style>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen-display.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen-display.tentative.html
new file mode 100644
index 00000000000000..64589e5267a195
--- /dev/null
+++ b/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen-display.tentative.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel=author href="mailto:masonf@chromium.org">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
+<link rel=match href="popup-initiallyopen-display-ref.tentative.html">
+
+<popup id=p1 initiallyopen>This is a popup, which should be open upon load</popup>
+<popup id=p2 initiallyopen>This is a second popup with initiallyopen, which should NOT be open upon load</popup>
+
+<style>
+  popup {
+    width: fit-content;
+    height: fit-content;
+    border: 1px solid;
+    padding: 1em;
+    background: white;
+    color: black;
+  }
+  #p1 {
+    top:0;
+  }
+  #p2 {
+    top:100px;
+  }
+</style>
+
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen.tentative.html
new file mode 100644
index 00000000000000..ac3e845b7ba86c
--- /dev/null
+++ b/html/semantics/interactive-elements/the-popup-element/popup-initiallyopen.tentative.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel=author href="mailto:masonf@chromium.org">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<popup id=p1 initiallyopen>This is a popup, which should be open upon load</popup>
+<popup id=p2 initiallyopen>This is a second popup with initiallyopen, which should NOT be open upon load</popup>
+<popup id=p3>Also not visible</popup>
+
+<script>
+  test(function(){
+    assert_true(p1.open,'initiallyopen should open the popup');
+    assert_true(p1.hasAttribute('initiallyopen'));
+    assert_true(p1.initiallyOpen,'initiallyopen should be reflected in the IDL attribute');
+    assert_false(p2.open, 'Only the first popup with initiallyopen should be open on load');
+    assert_true(p2.hasAttribute('initiallyopen'),'initiallyopen should be present/true, even if not opened');
+    assert_true(p2.initiallyOpen,'initiallyopen should be present/true, even if not opened');
+
+    assert_false(p3.open);
+    p3.setAttribute('initiallyopen','');
+    assert_false(p3.open, 'Changing initiallyopen should not affect open status');
+    assert_true(p3.hasAttribute('initiallyopen'));
+    assert_true(p3.initiallyOpen,'initiallyopen should still reflect to IDL');
+
+    p1.removeAttribute('initiallyopen');
+    assert_true(p1.open,'removing initiallyopen should not close the popup');
+    assert_false(p1.hasAttribute('initiallyopen'),'...but it should reflect to IDL');
+  }, "The initiallyopen attribute should affect page load only");
+</script>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-inside-display-none.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-inside-display-none.tentative.html
index 745403da5ff356..0b9fcc5427caf5 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-inside-display-none.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-inside-display-none.tentative.html
@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <link rel=author href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <link rel=match href="popup-hidden-display-ref.tentative.html">
 
 No popup should be displayed here.<p>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-invoking-attribute.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-invoking-attribute.tentative.html
index 2b5eaad51dd281..1ff60fd7f9b3ad 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-invoking-attribute.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-invoking-attribute.tentative.html
@@ -2,7 +2,7 @@
 <meta charset="utf-8" />
 <title>Popup invoking attribute</title>
 <link rel="author" href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="/resources/testdriver.js"></script>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss-on-scroll.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss-on-scroll.tentative.html
index 28dd4f4d187320..4f14dd7ae774b3 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss-on-scroll.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss-on-scroll.tentative.html
@@ -3,7 +3,7 @@
 <meta charset="utf-8" />
 <title>Popup light dismiss on scroll</title>
 <link rel="author" href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss.tentative.html
index c017893f1386c0..99781809c8e528 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-light-dismiss.tentative.html
@@ -2,7 +2,7 @@
 <meta charset="utf-8" />
 <title>Popup light dismiss behavior</title>
 <link rel="author" href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="/resources/testdriver.js"></script>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-open-display.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-open-display.tentative.html
index 7c5bf44b779470..792cc09d071a69 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-open-display.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-open-display.tentative.html
@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <link rel=author href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <link rel=match href="popup-open-display-ref.tentative.html">
 
 <popup>This is a popup</popup>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-open-overflow-display.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-open-overflow-display.tentative.html
index b2780b7af36231..ebe95d8544daf1 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-open-overflow-display.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-open-overflow-display.tentative.html
@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <link rel=author href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <link rel=match href="popup-open-overflow-display-ref.tentative.html">
 
 <div id=container>
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-shadow-dom.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-shadow-dom.tentative.html
index 96289a5e8f504c..516b6ead1b9ec2 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-shadow-dom.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-shadow-dom.tentative.html
@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <link rel="author" href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 
diff --git a/html/semantics/interactive-elements/the-popup-element/popup-stacking.tentative.html b/html/semantics/interactive-elements/the-popup-element/popup-stacking.tentative.html
index 4fccaa7c69a435..88e935056316d1 100644
--- a/html/semantics/interactive-elements/the-popup-element/popup-stacking.tentative.html
+++ b/html/semantics/interactive-elements/the-popup-element/popup-stacking.tentative.html
@@ -1,7 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <link rel="author" href="mailto:masonf@chromium.org">
-<link rel=help href="https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Popup/explainer.md">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 
