I think the spec should include some examples and expected outcomes. It’s not clear to me what the spec intends to happen for each of the following:

const url = 'data:text/javascript,export default "hello"';

import(url);
import(url, { assert: { type: 'javascript' } });
import(url, { assert: { type: '' } });
import(url, { assert: { type: null } });
import(url, { assert: { type: undefined } });

And if 'javascript' is the default type, but users aren’t allowed to specify it, there should be an explanation as to why. I think the explanation could be as simple as “because we want to allow potential future non-JavaScript module formats to not need type assertions.” That explanation would beg the question as to why the default type is 'javascript', but you could also explain that should there be non-JavaScript types allowed to do without an assertion, the default type might be renamed or changed to undefined or null.