Basically, content written for the web should be as short as possible and as simple as possible.

This will make sure that it can be quickly understood by people in all kinds of situations:

Tutorials and Articles on Writing Copy for the Web

Writing for the Web
Writing for the web 101
Copywriting case study: Bytecon
In this case study, I compressed a multi-page site into a single-pager that delivers a clean, concise message.
Case study: Business Improvement Network
Refreshing the site of this business networking organisation involved a new clean design and improvements to copy and information architecture
Case study: Foruse
A new clean look and a simpler tone of voice for the site of a usabilty engineering company

Other resources

"Learn to Write!"
Introuductory article on A List Apart.
A Guide to Writing Well
Josh Sowin's extensive article on Fire & Knowledge is well compiled and contains some useful information for web writing.
Brilliant Blogging Brings Big Benefits
Blogging is an excellent tool with benefits which feed into pretty much every area of online marketing. Kevin Gibbons explains the 5 reasons why you should be blogging.
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Articles + tutorials in Copywriting

Index: Copywriting for the Web
Tutorials and Articles on Writing Copy for the Web
Writing for the Web
General introduction to critical techniques for writing for the web
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