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+++ b/testing/web-platform/tests/css/css-anchor-position/position-fallback-dynamic.html
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+<!DOCTYPE html>
+<title>CSS Anchor Positioning Test: Dynamically change position via position-fallback property</title>
+<link rel="help" href="https://drafts.csswg.org/css-anchor-1/#fallback">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  body { margin: 0; }
+
+  @position-fallback --fallback1 {
+    @try {
+      left: anchor(--a1 right);
+    }
+  }
+  #anchor {
+    anchor-name: --a1;
+    width: 100px;
+    height: 100px;
+  }
+  #anchored {
+    position: absolute;
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div id="anchor"></div>
+<div id="anchored"></div>
+<script>
+  test(() => {
+    assert_equals(anchored.offsetLeft, 0);
+  }, "Initial static left position is 0");
+
+  test(() => {
+    anchored.style.positionFallback = "--fallback1";
+    assert_equals(anchored.offsetLeft, 100);
+  }, "Left position set to right edge of anchor with @position-fallback");
+</script>