Writing for the web for you might merely consist of putting your writing on your website for your family and friends to see. If you have a known audience who regularly reads your writing and you don't care whether search engines find your writing or not, have at it. Just write in bliss.
If one of your purposes in writing for the web consists of bringing traffic to your website, it'd serve you well to learn the ins and outs of search engine optimization. This is where you write while using words that will attract the right audience. There are several good books out there that will tell you the hows and the whys.
Writing right could make all the difference in the amount of traffic visiting your site.
Tip of the Day: Read a book on search engine optimization. You'll be glad you did.