How much should a UX professional know about front-end development?

Apologies if this is too much of a fuzzy question, I was wondering what people's thoughts were regarding the level of front-end development knowledge a UX professional (Experience Architect, Usability designer, Experience Designer - call it what you will) should have.

I'm guess I'm talking about situations where the two disciplines are handled by separate teams (UX team handle wireframes/designs/user journey and the front-end team actually do the coding).

Do you think the UX team can make do with a general level of front-end knowledge (basic understanding of HTML/CSS), or is it important for them to know the nitty gritty (semantic page structure, optimisation techniques, impact of JavaScript, graceful degradation/progressive enhancement, accessibility etc)?

Should a UX team be able to imagine how their designs/wireframes will be interpreted by the front-end team and what mark-up/technologies are likely to be used?

Does only having a limited knowledge of front-end development make for poorer UX?

Are the finer points of FE development solely the concern of the front-end devs, or should it be a consideration for the UX team as well?

As I said, I know it's a bit of a fuzzy question, but I'm just curious as to how people view the respective skills of FE and UX teams, and how much blurring there should ideally be between the two.