Well, a JS string is just a series of UTF-16 code-units (per ES5, there is no impl choice here), so there isn't really any encoding pre-se (and isn't necessarily a UTF-16 string, per the spec's definition thereof, as lone surrogates are valid). The fact that that works is more a testament to the the DOM being UTF-16 than JS.
(On the other hand, I'm sure you knew that. But probably there are people reading your comment who didn't. :))
