To address the use cases which aren't entirely covered by the proposed :context pseudo-class and improve performance for JS libraries, we may need to define a new scoped method.

IRC discussion in #whatwg:
<Lachy> othermaciej, I'm not yet convinced we need queryScopedSelector as well as this.
<othermaciej> Lachy: I think there are two reasons we may still want it:
<othermaciej> 1) otherwise JS libraries have to do rewriting of their incoming selectors, which is both slower and more error-prone in JS code than in native code
<othermaciej> 2) without a way to access those syntax features directly, it becomes harder for authors to ever switch off the library wrappers to the native version, even once all browsers support it

http://krijnhoetmer.nl/irc-logs/whatwg/20080711#l-23

There are also some use cases that aren't addressed by the current :context proposal, however, this is not intended to be a replacement for :context.  It is expected that the solution will be usable in conjunction with :context, but differently from the existing methods.

See the appendix at the end of this email
http://lists.w3.org/Archives/Public/public-webapi/2008May/0057.html

And a proposed solution to the parsing problem
http://lists.w3.org/Archives/Public/public-webapi/2008May/0058.html