wanting/needing to learn a new web language...

[jeffmc21]

I am relatively comfortable with HTML and Flash, especially in respect to building websites and web apps. I'm wanting to move to the next step, I'm just not very sure what that step should be. Does anyone have any suggestions as to what coding language or web developer's tool that I should start learning to increase my effectiveness as a web builder? And why choose that over something else?

You're opinions are appreciated.

[ray326]

Semantic HTML, CSS and Javascript/DOM because they are the fundamental client side building blocks.

[Kravvitz]

How well do you really know HTML? For example, do you know the two reasons you should use a complete doctype? Do you know how to use <label>s properly? Do you know what "semantic markup means?

If you don't know CSS, I would recommend that you learn it.

Adding a touch of style (http://www.w3.org/MarkUp/Guide/Style)
http://www.cssbasics.com/
http://www.htmldog.com/guides/
Integrated Web Design: CSS Beyond the Retrofit (http://www.informit.com/articles/article.asp?p=169546&rl=1)
BrainJar.com: Using Style Sheets (http://www.brainjar.com/css/using/)
Web Design 101 Articles (http://www.digital-web.com/types/web_design_101/)