diff --git a/lib/DOM/Element.php b/lib/DOM/Element.php
index 8af4a39..1198678 100644
--- a/lib/DOM/Element.php
+++ b/lib/DOM/Element.php
@@ -63,28 +63,7 @@ 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) {
-            $this->_classList->value = $value;
-        } else {
-            try {
-                parent::setAttribute($name, $value);
-            } catch (\DOMException $e) {
-                // The attribute name is invalid for XML
-                // Replace any offending characters with "UHHHHHH" where H are the
-                //   uppercase hexadecimal digits of the character's code point
-                $this->ownerDocument->mangledAttributes = true;
-                $name = $this->coerceName($name);
-                parent::setAttribute($name, $value);
-            }
-            if ($name === "id") {
-                $this->setIdAttribute($name, true);
-            }
-        }
+        $this->setAttributeNS(null, $name, $value);
     }
 
     public function setAttributeNS($namespaceURI, $qualifiedName, $value) {
@@ -93,7 +72,9 @@ class Element extends \DOMElement {
             $namespaceURI = trim($namespaceURI);
         }
         $qualifiedName = trim($qualifiedName);
-
+        if ($namespaceURI === null && ($this->namespaceURI ?? Parser::HTML_NAMESPACE) === Parser::HTML_NAMESPACE && !$this->hasAttributeNS($namespaceURI, $qualifiedName)) {
+            $qualifiedName = trim(strtolower($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/TreeBuilder.php b/lib/TreeBuilder.php
index ace1ae4..c123c9b 100644
--- a/lib/TreeBuilder.php
+++ b/lib/TreeBuilder.php
@@ -359,14 +359,6 @@ class TreeBuilder {
                     return true;
                 })());
 
-                // If attribute name coercison has occurred at some earlier point,
-                //   we must coerce all attributes on html and body start tags in
-                //   case they are relocated to existing elements
-                if ($token instanceof StartTagToken && $this->DOM->mangledAttributes && in_array($token->name, ["html", "body"])) {
-                    foreach ($token->attributes as $attr) {
-                        $attr->name = $this->coerceName($attr->name);
-                    }
-                }
                 # 13.2.6.4. The rules for parsing tokens in HTML content
                 // OPTIMIZATION: Evaluation the "in body" mode first is
                 //   faster for typical documents
@@ -386,7 +378,10 @@ class TreeBuilder {
                                 # not, add the attribute and its corresponding value to that element.
                                 $top = $this->stack[0];
                                 foreach ($token->attributes as $a) {
-                                    if (!$top->hasAttributeNS(null, $a->name)) {
+                                    // If attribute name coercison has occurred at some earlier point,
+                                    //   we must coerce all attributes on html and body start tags in
+                                    //   case they are relocated to existing elements
+                                    if (!$top->hasAttributeNS(null, $this->DOM->mangledAttributes ? $this->coerceName($a->name) : $a->name)) {
                                         $top->setAttributeNS(null, $a->name, $a->value);
                                     }
                                 }
@@ -414,7 +409,10 @@ class TreeBuilder {
                                 $this->framesetOk = false;
                                 $body = $this->stack[1];
                                 foreach ($token->attributes as $a) {
-                                    if (!$body->hasAttributeNS(null, $a->name)) {
+                                    // If attribute name coercison has occurred at some earlier point,
+                                    //   we must coerce all attributes on html and body start tags in
+                                    //   case they are relocated to existing elements
+                                    if (!$body->hasAttributeNS(null, $this->DOM->mangledAttributes ? $this->coerceName($a->name) : $a->name)) {
                                         $body->setAttributeNS(null, $a->name, $a->value);
                                     }
                                 }
diff --git a/tests/cases/TestDOM.php b/tests/cases/TestDOM.php
index 0cbb241..7fe227d 100644
--- a/tests/cases/TestDOM.php
+++ b/tests/cases/TestDOM.php
@@ -122,4 +122,30 @@ class TestDOM extends \PHPUnit\Framework\TestCase {
             ["TEST:TEST", "TESTU00003ATEST"],
         ];
     }
+
+    /** @dataProvider provideNamespacedAttributeSettings */
+    public function testSetNamespoacedAttributes(?string $elementNS, ?string $attrNS, string $nameIn, string $nameOut): void {
+        $d = new Document;
+        $e = $d->createElementNS($elementNS, "test");
+        $e->setAttributeNS($attrNS, $nameIn, "test");
+        $this->assertTrue($e->hasAttributeNS($attrNS, $nameOut));
+    }
+
+    public function provideNamespacedAttributeSettings(): iterable {
+        return [
+            [null,                                 null,                            "test",        "test"],
+            [null,                                 null,                            "TEST",        "test"],
+            ["http://www.w3.org/1999/xhtml",       null,                            "test",        "test"],
+            ["http://www.w3.org/1999/xhtml",       null,                            "TEST",        "test"],
+            [null,                                 null,                            "test:test",   "testU00003Atest"],
+            [null,                                 null,                            "TEST:TEST",   "testU00003Atest"],
+            ["http://www.w3.org/1999/xhtml",       null,                            "test:test",   "testU00003Atest"],
+            ["http://www.w3.org/1999/xhtml",       null,                            "TEST:TEST",   "testU00003Atest"],
+            [null,                                 "http://www.w3.org/1999/xhtml",  "test:test",   "test"],
+            [null,                                 "http://www.w3.org/1999/xhtml",  "TEST:TEST",   "TEST"],
+            ["http://www.w3.org/1998/Math/MathML", null,                            "test",        "test"],
+            ["http://www.w3.org/1998/Math/MathML", null,                            "TEST",        "TEST"],
+            [null,                                 "http://www.w3.org/2000/xmlns/", "xmlns:xlink", "xlink"],
+        ];
+    }
 }