As a professional web designer, what are the essential techniques?
In my opinion, these are the must-learns.
But however, learning these and skills using these languages may only produce well-written codes, as a skilled C/C++ programmer does. The layout of a page, colour, and accessibility issues may not be well enough.
Now the question is that, besides these essential languages, what other things must a web designer learn?
First, you should read this: What should a developer know before building a public web site?
When you have done that, spend time surfing the web. Look at the design of sites that you visit and determine why you like it or not. In that I would stress that it is just as important to look at bad design as good.
One thing I would add is a good understanding of accessibility - important for designer and developer.
Most of the (good) web designers that I know have art degrees. And very few of them really know the technologies underlying a website.
Stackoverflow Archive:
As a web designer, you should probably be proficient with Photoshop and/or Illustrator as well. And I wouldn't say you need to know XHTML, you might get by fine by knowing HTML, especially with the rise of HTML5 and the fall of XHTML2
Learning a little bit about Apache helps (or IIS), but more importantly, understanding HTTP (mostly GET/POST) can really make you see the big picture. Knowing exactly what is happening from your users' browsers to your code lets you fully understand how everything works...which hopefully leads to smarter design.
Use the Firebug addon to Firefox to be able to see all the requests/responses and headers for sites...that way you can see the redirects, and cookies which make sessions work, etc.
You need an eye for design. I suspect you can learn a lot about colors, fonts, layouts and so forth but it really helps if you have a built-in sense of what makes for a good design.
You also need an understanding of the underlying technologies. It makes no sense to craft some incredible design if it's not implementable. So, you need a background and understanding of the limitations of the platform(s) you are designing for.
Finally, you need a driving desire not to make something "cool" but something usable.
That's very strange that most people sticked to just technologies in their answer.
To become a professional web designer you need art/media/psychology education behind you, or at the very least some art taste, sense of beautiness and predisposition to these things. For a pure techie who cannot choose a color theme for a site and cannot understand why some UI design could be bad for end users, the design career path is closed forever. That's the most of the coders/developers.
Try to learn it on its own, read books on UI design, computer interaction psychology, learn about colors and their effects. Try doing it for at least a year, then you will see whether it's "yours" or not. Then if it works you can decide to specialize here by visiting some trainings, courses or maybe making a degree at some university.
ADDED:
Programmer's "design" example: http://www.flowershop.com/
Designer's design: http://www.reallywildflowers.co.uk/
Feel the difference...
This is a huge question because "design" has two meanings: "decoration" (graphic design) and "planning" (experience/UI design). I would argue they both influence "aesthetics" (a point often lost on people with formal training in either fine arts or computer science, who sometimes see "design" as something pretty slapped on top of a stack of engineering.)
If you're interested in improving your graphic design chops, you'll need an understanding and appreciation of basic design principles: contrast, proximity, color and typography. I also wouldn't hire a designer who can't draw perspective, still life, and human figures on a piece of paper with a pencil or charcoal, and without using rulers. I believe aesthetic and artistic ability can be learned (others argue aesthetic judgment is to some degree inborn). But at a certain level, you'll need to care a lot about how things look. (Litmus test: every designer in the world has a passionate opinion about Helvetica. Do you?)
All the good graphic designers I know spend some of their free time design doodling: a fun game is to take a piece of bad design (like a used-car ad or a Thai takeout menu) and make it into a piece of good design, but without altering the copy.
For graphic design, I'd recommend starting with a few books in design basics:
Taking a few classes in design basics and drawing at your local art school or community college wouldn't hurt.
I'd be hesitant to hire a graphic designer who couldn't wield Photoshop and Illustrator with proficiency, although someone with a good eye can learn these apps on the job.
I question whether experience design can be profitably learned from a book or class. It comes from working with all the parties who interact with a website: users, graphic designers, product managers, developers. IME really good experience designers begin with business objectives and then work with users or test groups to develop a mental model for delivering those objectives. They then break that model down into task domains (which might map to features) ... each of which gets its own user experience flow, wireframes and so on.
I guess I'd recommend these books:
There's no standard technologies for UX design, although you'll probably use Visio or OmniGraffle at some point. Study other designers' wireframes and flowcharts. Michael Angeles and Khoi Vinh have a lot of their thinking online.
In regards to web design in particular, the future seems to be frameworks and modular CMSes, with a rich interaction layer (ie. AJAX). Build a website in Drupal, Django, or Rails, just to see how those things work, and add a few Ajaxy elements to get a feel for those, too.
Finally, spend a lot of time on the web and pay attention. Think hard about every web experience you have. Good UX is invisible: It makes tasks so simple you don't have to think about them. So every time you find yourself trying to figure something out on a website, ask yourself how you'd make it better.