writing for the web

It's difficult to be concise on such a broad topic.  One could be writing marketing material on a retail site, for business to business, educational information, a personal or professional blog... the list goes on. For each type of writing, you'll find many opinions about style preferences.  Then, one has to determine what the definition of "effective" is for that website.  Is it to get more sales, more followers, more visits?  Is the writer trying to evoke emotion, providing instruction, requesting participation? 

The one concise tip I can give you is definitely grammar related:

Always make sure there are no grammar or spelling errors.  As the number of errors increases, the writer's credibility decreases. 

I agree with Patty.  

However, I will add that very often readers will engage more with clearly written and concise straightfoward style.  So, say what you want to say, but don't get too wordy or academic in the process.