LibWeb: Add tests for animating unresolved properties

Author: https://github.com/mattco98
Commit: 6bf1a30bf5
Pull-request: https://github.com/SerenityOS/serenity/pull/23622
Issue: https://github.com/SerenityOS/serenity/issues/23609

Tests/LibWeb/Text/input/css/animating-unresolved-properties-crash.html

@@ -0,0 +1,26 @@
+<!-- https://github.com/SerenityOS/serenity/issues/23609 -->
+<style>
+    :root {
+        --accent: #00f;
+    }
+
+    @keyframes myMove {
+        0% {
+            border-top: 2px solid var(--accent);
+        }
+    }
+
+    div {
+        animation: myMove 1s infinite;
+    }
+</style>
+<div></div>
+<script src="../include.js"></script>
+<script>
+    asyncTest(done => {
+        requestAnimationFrame(() => {
+            println("PASS! (Didn't crash)");
+            done();
+        });
+    });
+</script>