diff --git a/Userland/Libraries/LibWeb/DOM/CharacterData.idl b/Userland/Libraries/LibWeb/DOM/CharacterData.idl
index 4a67a12cd3d..a577a8c9e22 100644
--- a/Userland/Libraries/LibWeb/DOM/CharacterData.idl
+++ b/Userland/Libraries/LibWeb/DOM/CharacterData.idl
@@ -6,7 +6,8 @@ interface CharacterData : Node {
     readonly attribute Element? nextElementSibling;
     readonly attribute Element? previousElementSibling;
 
-    // FIXME: This should come from a ChildNode mixin
+    // FIXME: These should come from a ChildNode mixin
+    [CEReactions, Unscopable] undefined before((Node or DOMString)... nodes);
     [CEReactions, Unscopable, ImplementedAs=remove_binding] undefined remove();
 
 };
diff --git a/Userland/Libraries/LibWeb/DOM/ChildNode.h b/Userland/Libraries/LibWeb/DOM/ChildNode.h
index 559ff33fe35..7c8414a3eef 100644
--- a/Userland/Libraries/LibWeb/DOM/ChildNode.h
+++ b/Userland/Libraries/LibWeb/DOM/ChildNode.h
@@ -6,12 +6,51 @@
 
 #pragma once
 
+#include <LibWeb/DOM/ExceptionOr.h>
+#include <LibWeb/DOM/NodeOperations.h>
+
 namespace Web::DOM {
 
 // https://dom.spec.whatwg.org/#childnode
 template<typename NodeType>
 class ChildNode {
 public:
+    // https://dom.spec.whatwg.org/#dom-childnode-before
+    ExceptionOr<void> before(Vector<Variant<NonnullRefPtr<Node>, String>> const& nodes)
+    {
+        auto* node = static_cast<NodeType*>(this);
+
+        // 1. Let parent be this’s parent.
+        auto* parent = node->parent();
+
+        // 2. If parent is null, then return.
+        if (!parent)
+            return {};
+
+        // 3. Let viablePreviousSibling be this’s first preceding sibling not in nodes; otherwise null.
+        auto viable_previous_sibling = viable_previous_sibling_for_insertion(nodes);
+
+        // 4. Let node be the result of converting nodes into a node, given nodes and this’s node document.
+        auto node_or_exception = convert_nodes_to_single_node(nodes, node->document());
+        if (node_or_exception.is_exception())
+            return node_or_exception.exception();
+
+        auto node_to_insert = node_or_exception.release_value();
+
+        // 5. If viablePreviousSibling is null, then set it to parent’s first child; otherwise to viablePreviousSibling’s next sibling.
+        if (!viable_previous_sibling)
+            viable_previous_sibling = parent->first_child();
+        else
+            viable_previous_sibling = viable_previous_sibling->next_sibling();
+
+        // 6. Pre-insert node into parent before viablePreviousSibling.
+        auto result = parent->pre_insert(node_to_insert, viable_previous_sibling);
+        if (result.is_exception())
+            return result.exception();
+
+        return {};
+    }
+
     // https://dom.spec.whatwg.org/#dom-childnode-remove
     void remove_binding()
     {
@@ -27,6 +66,32 @@ public:
 
 protected:
     ChildNode() = default;
+
+private:
+    RefPtr<Node> viable_previous_sibling_for_insertion(Vector<Variant<NonnullRefPtr<Node>, String>> const& nodes) const
+    {
+        auto* node = static_cast<NodeType const*>(this);
+
+        while (auto* previous_sibling = node->previous_sibling()) {
+            bool contained_in_nodes = false;
+
+            for (auto const& node_or_string : nodes) {
+                if (!node_or_string.template has<NonnullRefPtr<Node>>())
+                    continue;
+
+                auto node_in_vector = node_or_string.template get<NonnullRefPtr<Node>>();
+                if (node_in_vector.ptr() == previous_sibling) {
+                    contained_in_nodes = true;
+                    break;
+                }
+            }
+
+            if (!contained_in_nodes)
+                return previous_sibling;
+        }
+
+        return nullptr;
+    }
 };
 
 }
diff --git a/Userland/Libraries/LibWeb/DOM/DocumentType.idl b/Userland/Libraries/LibWeb/DOM/DocumentType.idl
index 531cb3a2593..fa3ec8697e5 100644
--- a/Userland/Libraries/LibWeb/DOM/DocumentType.idl
+++ b/Userland/Libraries/LibWeb/DOM/DocumentType.idl
@@ -4,7 +4,8 @@ interface DocumentType : Node {
     readonly attribute DOMString publicId;
     readonly attribute DOMString systemId;
 
-    // FIXME: This should come from a ChildNode mixin
+    // FIXME: These should come from a ChildNode mixin
+    [CEReactions, Unscopable] undefined before((Node or DOMString)... nodes);
     [CEReactions, Unscopable, ImplementedAs=remove_binding] undefined remove();
 
 };
diff --git a/Userland/Libraries/LibWeb/DOM/Element.idl b/Userland/Libraries/LibWeb/DOM/Element.idl
index 41d664994d9..91a0e56d2b1 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.idl
+++ b/Userland/Libraries/LibWeb/DOM/Element.idl
@@ -54,7 +54,8 @@ interface Element : Node {
     readonly attribute long clientWidth;
     readonly attribute long clientHeight;
 
-    // FIXME: This should come from a ChildNode mixin
+    // FIXME: These should come from a ChildNode mixin
+    [CEReactions, Unscopable] undefined before((Node or DOMString)... nodes);
     [CEReactions, Unscopable, ImplementedAs=remove_binding] undefined remove();
 
 };