I'm a little late on getting to this one, but wanted to post before I forgot because it is a great topic for early and mid-career web designers. Matt (emenel) reacted to Mark Boulton's thoughts about whether or not designers needed to know html.
Matt summarizes what he believes:
As a designer there is a continuum you travel over the course of your career. It starts with making - crafting objects in your selected medium - and progresses through a point where you internalize your craft and can design without making. Even at that point, most designers continue to make things. It helps solidify design choices, and keeps you in touch with changes in the medium. Career wise, at this point you might be an art director, creative director, or design director… Before that point it is imperative that designers engage with their medium and craft - wether it be websites, clothes, telephones, chairs, or responsive environments.
Matt's post started with an observation of fashion designers, through watching Project Runway. These designers are judged not only on concept, but also on execution, or inability to execute. I think this is one profession in particular where the craft of constructing and prototyping are important. You see it when looking at process. Some designers make cartoonish sketches and others beautiful concept drawings. But in the end, the concept can't stand alone if they're judged equally on execution.
Learning html for Mosaic is one of the reasons I was drawn to the web. It was like taking a toaster apart only to learn how to put it back together. I took my first job basically to apprentice, even though I would be learning most of the HTML techniques on my own and through mailing lists like ALA. All that time and experience helps me every day that I draw a UI schematic. The immersion over time is what makes it become part of your language, and gives you the confidence to design without limitations.
I don't think that everyone needs to create production-quality html. I agree with Matt on the main point here. UX designers benefit from developing a working knowledge of the craft of html and css, and should at least put in some time working on prototypes to understand the capabilities of the medium. Even if new designers only work on personal projects, the experience helps them if they never use that skill in producing a deliverable. At best, it gives one the ability to personally demonstrate interactivity with a prototype resembling a working product if they have to.
The biggest point of contention may be whether they need to maintain this ability over time. I don't necessarily think that's true. I, like many other UX designers I know, are familiar enough with HTML to prototype, but are good enough at sketching or with using tools to demonstrate ideas so that someone better suited to building the interactive prototype can execute. That's not to say that we'll all let those skills drop by the wayside as our careers mature. I certainly try to prevent that from happening, which is one of the reasons I blog. But there are resources that can prototype faster and better once we select and refine an idea enough. In the end, execution for me means communicating a refined concept, and doing that doesn't always mean making html or writing code. But the process certainly is informed by understanding it.