What are the things you need to be good at to be employable as a Web developer?
Are there specific platforms and languages you need to know?
How do you get the training?

I think following things are necessary:
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1. You should be good at HTML
2. You should be familiar with JavaScript
3. You should be familiar with CSS
4. Yous should be familiar with at least one of the
server side scripting language like PHP, JSP,
ASP.
5. You should know about browser technology,
else will be endup suffering from browser war.

Like DangerDev said, all of those languages are critical. A lot of people think that CSS isn't really necessary to learn because you don't NEED it, but try developing a multi-page site without it.
The only things Dev didn't mention that you may want to know are Flash, SEO, and possibly have a familiarity with MySQL and Apache. But, you can get plenty of design jobs that don't require these skills.

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