XMLDocument.createElement() uppercases element names

[BusfahrerWalter]

Describe the bug

XMLDocument.createElement() uppercases element names even though XML element names are case-sensitive.

When parsing a document as text/xml, calling createElement('myNode') results in an element whose nodeName and tagName are "MYNODE" instead of "myNode".

StackBliz

To Reproduce

1. Create an XML document:

   const doc = new DOMParser().parseFromString('<root/>', 'text/xml');

2. Create an element:

   const element = doc.createElement('myNode');

3. Inspect the element:

   console.log(element.nodeName);
   console.log(element.tagName);

Current output:

MYNODE
MYNODE

Expected behavior

The element name should be preserved exactly as provided when using an XMLDocument.

Expected output:

myNode
myNode

This is the behavior defined for XML documents, where element names are case-sensitive.

Device and details:

• OS: Ubuntu 20.04.0 LTS
• Node version: 22.22.3
• Package version: 20.11.1

Additional context

This appears to be HTML createElement() behavior being applied to XMLDocument. Parsing existing XML preserves the original case correctly, but creating new elements programmatically does not.