Have you ever tried to read your local newspaper online? How about the New York Times or the Wall Street Journal? It just isn't the same. Do you know why? Because articles meant for print don't translate well to the web, and the rules that apply to writing content for the internet are different than those for print. What constitutes quality content offline does not necessarily constitute quality content online.
How the web is different:
Text is hard to read
Typical computer monitors have a resolution of 96 dpi (dots per inch). Compare that with a printout from a laser printer that has a resolution of 600 dpi, or a magazine page that can be upwards of 2400 dpi, and it's not hard to figure out why the text on a computer places a strain on the eyes. According to the book Hot Text, Web Writing That Works, by Jonathan and Lisa Price, "because text is more difficult to read on-screen, people often read slower, comprehend less, recall less, and do less in response."
Words can be linked to other pages and sources
The closest thing you're going to get to a link in a newspaper is when a story is split into two sections and you're told the story is continued on page 9. But when it comes to the web, words and images can be linked to other web pages, photos, videos, sounds, and a myriad of other things. Being able to link is the primary tool that web writers can take advantage of that print writers don't have at their disposal.
How you should write:
Because of the strain placed on readers when reading on a computer screen, you can't expect them to read a 5,000 word article. Research has shown that most readers tend to scan an article before reading. Articles longer than 1,000 words will likely turn off your audience and result in few people reading your article (and therefore fewer visitors coming back to your site in the future). You should condense your writing to include only the most crucial points and eliminate everything else. Writing successfully for the web forces you to present only the necessary content and leave the rest out.
Utilize headings and lists
Since people tend to scan web articles as opposed to reading them from the first word to the final word, you should make it easy for them to find what they're looking for by using headings, bold type, and lists. A great way to turn a print article into a readable web article is to transform it into a top 10 list. Lists make it easy for readers to scan and read only what they are interested in.
Use plenty of links and make them obvious
Since linking is the primary advantage of a web writer, it should be used early and often. Linking allows you to provide the reader with a roadmap of information. With your article as a starting point, your reader should be able to find more information about any and all topics discussed in the article. Common things to link to include reference pages, news sources, audio and video, forums, and applications that will enhance the reading experience. The best thing about links is that the user can choose which ones to follow and which ones to ignore. That allows you to reference something without citing the entire thing as you would have to in a print article.
One of the most important things to remember when linking is to make it visually obvious that a section of text is a link. Five years ago it was common place to use the standard blue underlined text for linking, but as the web has evolved, most designers have abandoned that style for better looking links. Site designers can still make links obvious by consistently using a different color than standard text and by providing a hover effect, such as underlining the link and changing it's color, when users place their cursor over the link. This subconsciously tells them that the text is a link.
Write with the search engines in mind
Since much of the content on the web is found via search, it makes sense to write with the search engines in mind. No, this doesn't mean that you should stuff your articles with keywords to the point where they are barely readable. But it does mean that you should write titles and headings that actually convey what your article discusses. For example, this article could be called "Content Evolution" or "Digital Distribution." If it were a magazine article, those titles or other titles might have been more appealing, but they don't capture the essence of the article, which is "Writing for the Web." If someone were to search for an article on writing online, they'd likely use a phrase like "how to write for the web" or "writing practices for the web" which would turn up this article, but probably wouldn't turn up an article with one of those other titles.
Resource: Price, Jonathan, and Lisa Price. Hot Text - Web Writing That Works. Indiana: New Riders, 2002.