diff --git a/Userland/Libraries/LibWeb/DOM/DOMImplementation.cpp b/Userland/Libraries/LibWeb/DOM/DOMImplementation.cpp
index d233b1fd20..1f6aa5d69d 100644
--- a/Userland/Libraries/LibWeb/DOM/DOMImplementation.cpp
+++ b/Userland/Libraries/LibWeb/DOM/DOMImplementation.cpp
@@ -89,9 +89,11 @@ NonnullRefPtr<Document> DOMImplementation::create_html_document(const String& ti
 }
 
 // https://dom.spec.whatwg.org/#dom-domimplementation-createdocumenttype
-NonnullRefPtr<DocumentType> DOMImplementation::create_document_type(String const& qualified_name, String const& public_id, String const& system_id)
+ExceptionOr<NonnullRefPtr<DocumentType>> DOMImplementation::create_document_type(String const& qualified_name, String const& public_id, String const& system_id)
 {
-    // FIXME: Validate qualified_name.
+    auto result = Document::validate_qualified_name(qualified_name);
+    if (result.is_exception())
+        return result.exception();
     auto document_type = DocumentType::create(document());
     document_type->set_name(qualified_name);
     document_type->set_public_id(public_id);
diff --git a/Userland/Libraries/LibWeb/DOM/DOMImplementation.h b/Userland/Libraries/LibWeb/DOM/DOMImplementation.h
index 391ee2820e..4dc03900ad 100644
--- a/Userland/Libraries/LibWeb/DOM/DOMImplementation.h
+++ b/Userland/Libraries/LibWeb/DOM/DOMImplementation.h
@@ -30,7 +30,7 @@ public:
     // FIXME: Add optional DocumentType once supported by IDL
     ExceptionOr<NonnullRefPtr<Document>> create_document(const String&, const String&) const;
     NonnullRefPtr<Document> create_html_document(const String& title) const;
-    NonnullRefPtr<DocumentType> create_document_type(String const& qualified_name, String const& public_id, String const& system_id);
+    ExceptionOr<NonnullRefPtr<DocumentType>> create_document_type(String const& qualified_name, String const& public_id, String const& system_id);
 
     // https://dom.spec.whatwg.org/#dom-domimplementation-hasfeature
     bool has_feature() const { return true; }
diff --git a/Userland/Libraries/LibWeb/DOM/Document.cpp b/Userland/Libraries/LibWeb/DOM/Document.cpp
index c36ce5f5d2..95cc0ffe88 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Document.cpp
@@ -1390,4 +1390,54 @@ bool Document::is_valid_name(String const& name)
     return true;
 }
 
+// https://dom.spec.whatwg.org/#validate
+ExceptionOr<Document::PrefixAndTagName> Document::validate_qualified_name(String const& qualified_name)
+{
+    if (qualified_name.is_empty())
+        return InvalidCharacterError::create("Empty string is not a valid qualified name.");
+
+    Utf8View utf8view { qualified_name };
+    if (!utf8view.validate())
+        return InvalidCharacterError::create("Invalid qualified name.");
+
+    Optional<size_t> colon_offset;
+
+    bool in_name = false;
+
+    for (auto it = utf8view.begin(); it != utf8view.end(); ++it) {
+        auto code_point = *it;
+        if (code_point == ':') {
+            if (colon_offset.has_value())
+                return InvalidCharacterError::create("More than one colon (:) in qualified name.");
+            colon_offset = utf8view.byte_offset_of(it);
+            continue;
+        }
+        if (in_name) {
+            if (!is_valid_name_start_character(code_point))
+                return InvalidCharacterError::create("Invalid start of qualified name.");
+            in_name = false;
+            continue;
+        }
+        if (!is_valid_name_character(code_point))
+            return InvalidCharacterError::create("Invalid character in qualified name.");
+    }
+
+    if (!colon_offset.has_value())
+        return Document::PrefixAndTagName {
+            .prefix = {},
+            .tag_name = qualified_name,
+        };
+
+    if (*colon_offset == 0)
+        return InvalidCharacterError::create("Qualified name can't start with colon (:).");
+
+    if (*colon_offset >= (qualified_name.length() - 1))
+        return InvalidCharacterError::create("Qualified name can't end with colon (:).");
+
+    return Document::PrefixAndTagName {
+        .prefix = qualified_name.substring_view(0, *colon_offset),
+        .tag_name = qualified_name.substring_view(*colon_offset + 1),
+    };
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/DOM/Document.h b/Userland/Libraries/LibWeb/DOM/Document.h
index 19c382bd14..3d45e33f0f 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.h
+++ b/Userland/Libraries/LibWeb/DOM/Document.h
@@ -318,6 +318,12 @@ public:
 
     static bool is_valid_name(String const&);
 
+    struct PrefixAndTagName {
+        FlyString prefix;
+        FlyString tag_name;
+    };
+    static ExceptionOr<PrefixAndTagName> validate_qualified_name(String const& qualified_name);
+
 private:
     explicit Document(const AK::URL&);