diff --git a/lib/DOM/Document.php b/lib/DOM/Document.php
index 4d60f6e..5f00bb1 100644
--- a/lib/DOM/Document.php
+++ b/lib/DOM/Document.php
@@ -36,6 +36,9 @@ class Document extends \DOMDocument {
     }
 
     public function createAttribute($name) {
+        // Normalize the attribute name per modern DOM specifications.
+        $name = strtolower(trim($name));
+
         try {
             return parent::createAttribute($name);
         } catch (\DOMException $e) {
@@ -49,6 +52,12 @@ class Document extends \DOMDocument {
     }
 
     public function createAttributeNS($namespaceURI, $qualifiedName) {
+        // Normalize the attribute name and namespace URI per modern DOM specifications.
+        if ($namespaceURI !== null) {
+            $namespaceURI = strtolower(trim($namespaceURI));
+        }
+        $qualifiedName = trim($qualifiedName);
+
         try {
             return parent::createAttributeNS($namespaceURI, $qualifiedName);
         } catch (\DOMException $e) {
@@ -62,6 +71,9 @@ class Document extends \DOMDocument {
     }
 
     public function createElement($name, $value = "") {
+        // Normalize the element name per modern DOM specifications.
+        $name = strtolower(trim($name));
+
         try {
             if ($name !== 'template') {
                 $e = parent::createElement($name, $value);
@@ -83,6 +95,13 @@ class Document extends \DOMDocument {
     }
 
     public function createElementNS($namespaceURI, $qualifiedName, $value = "") {
+        // Normalize the element name and namespace URI per modern DOM specifications.
+        if ($namespaceURI !== null) {
+            $namespaceURI = strtolower(trim($namespaceURI));
+            $namespaceURI = ($namespaceURI === Parser::HTML_NAMESPACE) ? null : $namespaceURI;
+        }
+        $qualifiedName = trim($qualifiedName);
+
         try {
             if ($qualifiedName !== 'template' || $namespaceURI !== null) {
                 $e = parent::createElementNS($namespaceURI, $qualifiedName, $value);
diff --git a/lib/DOM/Element.php b/lib/DOM/Element.php
index a6cf4b1..8a4b868 100644
--- a/lib/DOM/Element.php
+++ b/lib/DOM/Element.php
@@ -63,6 +63,9 @@ class Element extends \DOMElement {
     }
 
     public function setAttribute($name, $value) {
+        // Normalize the attribute name per modern DOM specifications.
+        $name = strtolower(trim($name));
+
         // If setting a class attribute and classList has been invoked use classList to
         // set it.
         if ($name === 'class' && $this->_classList !== null) {
@@ -85,6 +88,12 @@ class Element extends \DOMElement {
     }
 
     public function setAttributeNS($namespaceURI, $qualifiedName, $value) {
+        // Normalize the attribute name and namespace URI per modern DOM specifications.
+        if ($namespaceURI !== null) {
+            $namespaceURI = strtolower(trim($namespaceURI));
+        }
+        $qualifiedName = trim($qualifiedName);
+
         // If setting a class attribute and classList has been invoked use classList to
         // set it.
         if ($qualifiedName === 'class' && $namespaceURI === null && $this->_classList !== null) {
diff --git a/lib/DOM/traits/Serialize.php b/lib/DOM/traits/Serialize.php
index e0ed9ff..ae51d71 100644
--- a/lib/DOM/traits/Serialize.php
+++ b/lib/DOM/traits/Serialize.php
@@ -37,7 +37,7 @@ trait Serialize {
 
         # 3. If the node is a template element, then let the node instead be the
         # template element’s template contents (a DocumentFragment node).
-        if ($node instanceof Element && $node->nodeName === 'template') {
+        if ($node instanceof TemplateElement) {
             $node = $node->content;
         }