fix(attributes): make removeAttr case-insensitive for HTML documents

htmlparser2 stores HTML attribute names in lowercase per the HTML spec.
removeAttribute was doing a case-sensitive Object.hasOwn check and delete,
which caused removeAttr('CLASS') to fail when the attribute was stored
as 'class'. Normalize the attribute name to lowercase before checking
and deleting to match htmlparser2's storage convention.

Fixes #5257

Author: AryaRizky

src/api/attributes.ts

@@ -838,9 +838,12 @@ export function val<T extends AnyNode>(
  * @param name - Name of the attribute to remove.
  */
 function removeAttribute(elem: Element, name: string) {
-  if (!(elem.attribs && Object.hasOwn(elem.attribs, name))) return;
+  // Normalize attribute name to lowercase; htmlparser2 stores HTML attribute
+  // names in lowercase per the HTML spec (case-insensitive attributes).
+  const normalizedName = name.toLowerCase();
+  if (!(elem.attribs && Object.hasOwn(elem.attribs, normalizedName))) return;
 
-  delete elem.attribs[name];
+  delete elem.attribs[normalizedName];
 }
 
 /**