The original issue report noted an incorrect HTML structure based on the PHP based rendering. Since the initial report, the JavaScript handling has changed and the inner div with the system-message ID removed. This PR should make the HTML rendered by the JS consistent with the PHP layout.
Test Instructions
Honestly, I don't remember where in Joomla we render messages by JavaScript. But, the rendered HTML should be consistent with the HTML in the layout file specified in the initial issue.
I'd say we use it in the CMS Installation and maybe with the JavaScript form validating