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

In the description of register, NAME is a custom element name and EXTENDS is a local-name.

However in the element registration algorithm, TYPE is the custom element name and NAME is a local name.

The spec is correct, there's alpha-renaming of TYPE/NAME and NAME/EXTENDS. It is just *really* confusing when reading the spec to have TYPE and NAME refer to two different things in slightly different contexts.

The Blink implementation uses TYPE for the thing that uniquely identifies the custom element. For custom tags, this is the tag name. For type extensions, this is the value of the "is" attribute.

NAME is always the tag name of the custom element. For custom tags, TYPE and NAME have the same value.