"NAME, the custom element name of the element being registered"

The HTML namespace is case insensitive but the SVG and XHTML namespace is not. How should we treat case for custom element names? A few options on the top of my head are:

1. Make names case sensitive. This would causes a bit of weirdness because the name custom-Foo could be registered but useless in the HTML namespace because it contains a capital letter.

2. Make names case insensitive. This would also cause weirdness in SVG and XHTML because custom-Foo and custom-foo could not both be custom elements even though they would be different types of elements.

3. Make names case sensitive if it "inherits" from SVGElement and case insensitive if it "inherits" from HTMLElement, but this causes increased complexity and still causes weirdness for XHTML.