If we have the query selector div > .hello.world > span, the current query selector parser produces the following result:

Parent(
    Tag([div]),
    And(
        Class([hello]),
        Parent(
            Class([world]),
            Tag([span])
        )
    )
)

This is equivalent to div > (.hello AND .world > span), which would be written as div.world > span.hello. This is a bit unexpected, as the spec would suggest the following as the result:

Parent (
    Tag([div]),
    Parent(
        And(
            Class([hello]),
            Class([world])
        ),
        Tag([span])
    )
)

I ran into this issue while making an attempt at fixing #22 but this seems to be a prerequisite to implementing child selectors. Since the parent combinator is associative, it's also possible to parse the query as left associative instead of right associative:

Parent (
    Parent (
        Tag([div]),
        And(
            Class([hello]),
            Class([world])
        ),
    ),
    Tag([span])
)

which seems to be easier for eventually implementing child selectors, since you can match a node to the right and its parent to the left. That assumes you have access to the parent though, which is part of how I had been planning on implementing the parent selector. Otherwise, you'd likely have to approach it top-down.

To fix this issue though, it seems like one would need to add a bit more complexity to the query selector parser to handle precedence. I believe this is what @kelko was running into with and and or operations in #22.