Abstract: "[CSS21]" isn't italicised.

"(often simply referred to as selector)" - the grammar sounds wrong; should it be "...as a selector"?

"it's easier to match" - seems more conventional to say "it is" instead. Also, ASCII ' vs some Unicode '-like symbol are used inconsistently.

"var cells = new Array();" - I'd say "var cells = [];" but maybe that's just personal preference. I'd also remove the "var rows = null;" and put the "var" inside the loop, though maybe that's bad because the scoping is weird.

"as described in [RFC2119]." vs "is that from Selectors [Selectors]." - should be consistent on whether the citation is part of the sentence.

"""object implementing the Foo interface "." - stray space at the end.

"return "http://example.org/test"" (and another couple of places in that section) - also probably personal preference, but I don't like dropping semicolons (except after }s).

"var <var>x</var>" - seems odd to do that for just a few of the variables. (It's still inconsistent with variables like svgImages that are referred to in the non-code text.)

"if("test" == prefix)" - the lack of space before the ( seems inconsistent with all the other code.

The interface definitions have inconsistent indentation of the method names.

"The selectAllElements() methods on the DocumentSelector interface ..." - says "in document order" twice.

I guess it's not necessary to say that exceptions thrown inside lookupNamespaceURI must propagate outwards to the selectElement caller - hopefully that's obvious or is defined elsewhere.

"When the lookupNamespaceURI() method is invoked with an empty string as the argument ... it must do either of the following:" - that 'must' is redundant with the two following 'must's, which is perhaps a little irritating when you're trying to extract a testable assertion for each 'must' in the document.

"User agents must handle prefixes case sensitively." - does 'handle' actually mean anything (to someone who knows more about this stuff than I do), given that it has already stated they must be passed case-preservingly to the NSResolver?

"var lis = document.selectAllElements("ul.nav>li"); for (var i = 0; i < items.length; i++) {" - s/items/lis/, and for the next example.

"[CaseMap] (Non-normative) Unicode Standard Annex #21 Case Mappings" - name isn't italicised.

"[CSS21]: ... T. [funny symbols]elik" looks like double-encoded UTF8. (The [Selectors] copy of the name is fine.)