SyntaxError: syntax error <XYZ MY TAG>
In what way can it influence the rendering of the page - i.e. the Browser doesn't recognize it
It gets added to the DOM. From there it can influence the rendering of the page.
It won't be ignored. It will be added to the DOM as an unknown element. (spec).
Since it is not valid JS, the JS engine will throw an exception.
What happens in HTML to an unknown element - does it affect the rendering of the page?