Handle xmlns on HTML elements correctly

The test for when to produce a parse error was not correct previously

lib/Parser/TreeConstructor.php

@@ -3541,7 +3541,7 @@ class TreeConstructor {
                             # corresponding cell in the fourth column. (This fixes the use of namespaced
                             # attributes, in particular lang attributes in the XML namespace.)
                             // DOMElement::setAttributeNS requires the prefix and local name be in one
-                            // string, so there is no need to separate the prefix and the local name here.
+                            //   string, so there is no need to separate the prefix and the local name here.
                             $a->namespace = self::FOREIGN_ATTRIBUTE_NAMESPACE_MAP[$a->name] ?? null;
                         }
                         # Insert a foreign element for the token, in the same namespace as the adjusted
@@ -4236,9 +4236,9 @@ class TreeConstructor {
             // NOTE: The specification is silent as to how to handle these
             //   attributes. We assume these bad attributes should be dropped,
             //   since they break the DOM when added
-            if ($attr->name === "xmlns" && $namespace !== $this->htmlNamespace && $attr->value !== $namespace) {
+            if ($attr->name === "xmlns" && $attr->namespace === Parser::XMLNS_NAMESPACE && $attr->value !== ($namespace ?? Parser::HTML_NAMESPACE)) {
                 $this->error(ParseError::INVALID_NAMESPACE_ATTRIBUTE_VALUE, "xmlns", $namespace);
-            } elseif ($attr->name === "xmlns:xlink" && $namespace !== $this->htmlNamespace && $attr->value !== Parser::XLINK_NAMESPACE) {
+            } elseif ($attr->name === "xmlns:xlink" && $attr->namespace === Parser::XMLNS_NAMESPACE && $attr->value !== Parser::XLINK_NAMESPACE) {
                 $this->error(ParseError::INVALID_NAMESPACE_ATTRIBUTE_VALUE, "xmlns:xlink", Parser::XLINK_NAMESPACE);
             } else {
                 $this->elementSetAttribute($element, $attr->namespace, $attr->name, $attr->value);

tests/cases/TestTreeConstructor.php

@@ -135,7 +135,7 @@ class TestTreeConstructor extends \PHPUnit\Framework\TestCase {
     }
 
     protected function patchTest(string $data, $fragment, array $errors, array $exp): array {
-        // When using the HTML namespace, xmlns attributes lose their namespace due to a PHP limitation
+        // When using the HTML namespace, xmlns attributes in the xmlns namespace lose their namespace URI due to a PHP limitation
         if ($this->ns) {
             for ($a = 0; $a < sizeof($exp); $a++) {
                 if (preg_match('/^\|\s+xmlns xmlns=/', $exp[$a])) {