Too often, the content of Websites is the lowest priority of the people with the money. But TV watchers have been waiting for 50 years for serious content, and the Web could become just another information sewer. In Webtext, the pitch more likely will focus on the strongest argument for buying the product, and ignore the rest. This means that you are wasting your readers' time unless you cut your text by the same amount. Every Webtext is really just half a dialogue, with the reader providing the other half.
Good writing is good writing whatever the medium.
The difference in Web writing is that good writing is also usually very brief writing. It's clear, concrete, easy to understand without being dumbed down. Grammar, spelling, punctuation and accurate diction are all critical in Web writing as in print. Too many people want to influence the look and design and content of such sites, and they often have no interest in the Web as a medium. Sometimes, alas, they have no interest in their customers either.