The web is built on a single foundation: HTML. Sure, CSS and Javascript have extended it greatly, but everything ultimately boils down to HTML. This is the lingua franca of the web.
Designers who start with Photoshop generally (and keep in mind I'm making a lot of generalizations here that don't necessarily apply to every web designer everywhere) are looking at the web the same way they look at a print brochure or flyer, and that's wrong. "Designing" for the web should take the medium into account, and I don't mean that in the sterotypical visual way. I mean that starting from HTML and building it up with CSS produces a clear, more efficient, more informed result. I mean it from the perspective of learning a foreign language by rote; repeating key phrases over and over without understanding what the individual sounds being uttered actually mean as compared to learning a language the old fashioned way; vocabulary, structure, grammar, colloquialisms, practice, practice, practice. Only someone who has done the latter can truly be fluent in a language.
The difference between graphic design and web design is specifically that there is an underlying language and grammar governing every piece of output. Some (poor) web designers get around their lack of understanding of this by producing full-page images and simply spitting out one big tag per page. This is the most extreme example of course, but there is a whole continuum from that all the way to developers who could not make a web page look attractive to save their lives. (For the record, I'm way far down at that far end of the spectrum.)
To reiterate that point; I am not a web designer. I'm not good enough at the, "making pages look pretty," part to rightly label myself that way. I do believe though, that the sweet spot on the continuum is a solid foundation from both ends: Someone with the aesthetic tastes to make something beautiful, but that understands that the best way to do it isn't by slicing and force fitting an ill-considered PSD design into
As an application developer, these are the designers I dream of working with. Someone who can take the output of my code and, with the appropriate classes and id's, mold it into a dozen different shapes and styles.