Looks like steps 3-5 of the prototype construction algorithm (Section 5) should be part of the main steps (aka need to be outdented).

Suspect it's supposed to read like this:

1. If EXTENDS is an invalid HTML tag name, throw an InvalidCharacterError exception.
2. If EXTENDS is a custom element name name, let BASE be the element prototype of the custom DOM element with the custom element name of EXTENDS
3. Otherwise:
    1. If EXTENDS is defined in HTML specification or other applicable specifications, let BASE be the interface prototype object for the element type corresponding to the HTML tag name of EXTENDS
    2.Otherwise, throw a NotSupportedError exception.
4. Create a new object that implements BASE
5. Let PROTOTYPE be this new object
6. If PROPERTIES is present and not undefined, define properties on PROTOTYPE using PROPERTIES