Writing for the web is a completely unique style of writing. Why? because our brains process online information completely differently to any other medium.
With web content, we read in an F-shaped pattern. We scan (not read-scan) from left to right, then go back to the left starting point and scan downwards.
Also, with effective web content, less is more. As Crawford Kilian said: 'Every sentence, every phrase, every word has to fight for it's life.' Everything on a web page has to earn it's right to be there. If it's not there by appointment, it shouldn't be there at all.
At The Writing Cooperative, Cynthia Marinakos has written a great piece that talks about the rules of effective web content and why our brains process online information so differently than any other type of reading.