New comments:

The term "first" is not defined (for |match()|). Specifically, it should say whether it is the first match in a depth-first pre-order traversal, or whether it is a breath-first traversal (depth-first pre-order makes the most sense, IMHO).

"In document order" could be more explicit.

There is no normative description of how |Function| objects in ECMAScript end up supporting XPathNSResolver.

I don't think "In ECMAScript bindings the nsresolver argument in both match and matchAll must be an optional argument." as a requirement makes sense. I think it would be better to phrase it as something like "In ECMAScript, if the nsresolver argument in an invocation of match() or matchAll() is omitted, UAs must handle the invocation as if the nsresolver argument was null." or something.

Also, the spec should say what should happen if the nsresolver argument is null.

If you really want to not use the term NodeList, I recommend defining StaticNodeList as:

typedef StaticNodeList NodeList;

...rather than duplicating the interface definition.

That is a non-normative note. It's also not clear where in that spec it is defined what the requirements on implementations and authors are.

Section 5 of Selectors.

There are three main reasons. One is that pseudo-elements can in certain cases refer to Elements (XBL2 in particular introduces this concept).

The second reason is that pseudo-elements are perfectly valid selectors, so if the script is just running through a list of selectors and checking each one for a match, it will not want valid selectors to throw an exception. Consider, e.g., if the input is coming from a user -- you want to distinguish "doesn't match any elements (because it can't)" from "is invalid syntax".

The selector "foo:not(foo)" will never match anything either, but it doesn't raise an exception. You don't want to require the implementation to look at the selector any more than it would for a normal CSS implementation.

The third reason is that you could have a selector that contains pseudo-elements and yet can match an element, for instance:

p, p::first-line

...can match an element (p).

Otherwise, looks good.