I really don't follow. A kick-ass interface designer can know nothing more than Photoshop. A front-end guru can be terrible at interface design. There's very little connection between interface design and the technologies used to implement it.
For starters, whatever you design in Photoshop typically looks 10-30% different in the browser when converted to HTML. That is significant enough to make the concept look very different than the actual browser interface implementation.
An interface designer who only operates in Photoshop thus cannot deliver accurate-looking concepts.
HTML isn't the culprit here. Any design done in a static medium is going to change when actually implemented on a device, for a platform. But it could just as easily be Flash as HTML. Good designers understand and account for the target medium (flexible sizes, variable fonts etc).
Also I reject your initial hypothesis. With a knowledgeable designer's design I can get a pixel perfect HTML implementation. If your design changes 30% just in the implementation then your designer doesn't understand the medium very well.
The best designers will know HTML+CSS+Javascript pretty intimately for a variety of reasons.
Can you name a few? I work with an absolutely brilliant designer who can't write a lick of HTML/JS/CSS however he intimately understands how his designs will translate to a browser.
Good interface designers should definitely know the basics of the technologies used to implement their creations, in the same way industrial designers are expected to have some understanding of materials and manufacturing processes. It's often said that designers thrive on constraints: a designer does his best work when he knows the limitations of his tools, and works with their limitations and defects to create something useful instead of pursuing some impossible level of perfection. Designers, too, have to ship, and they ship best when they know what's behind the facade.
contrast that to being able to rapidly iterate on the real code.
i don't mean iterate as in "code a feature, commit to the repo, send for review, do over". i mean the tweaking while coding.