Hang out on forums, #javascript, etc some time and see how many people have problems because their understanding of the DOM begins and ends with the jQuery API. I see it all the time. I've seen big-name sites bug out or slow to a crawl because some dev wasn't thinking about what was going on behind their $()'s.

The DOM API isn't very large/complicated, and it shouldn't be hard to MOSTLY memorize it. It isn't a "gotcha" question though, a good interviewer is not gonna ding the heck out of you if your forget bits of it (most web devs do spend most of their time with jQuery, YUI, etc after all) - it's about whether you understand what's going on behind the scenes.

I don't mind the need to research DOM-level APIs on the fly. In fact, I'd be just as impressed that they not only know what they need to look up to refresh their memory, but where to look. Where and how they look might say things about them as well.

After all, I often have to look up a rarely used method. Seems unfair to judge harshly if they do so.

Native API are probably easier to learn, just more cumbersome to use :)