Whether you're writing for print or the web, Strunk and White's The Elements of Style has plenty of good rules and principles you should follow. Point 17, "Omit needless words," is one of the best--you might not have a word limit on your blog or webpage, but no one will want to read 1,000 words when 300 words could get the message across as well or better. Point 2--which advocates the use of the "serial comma" or "Oxford comma"--is more flexible, but you'll appease the grammar Nazis if you follow it. (If you don't know what a serial comma is, consult your local grammar Nazi--they'll probably be more than happy to tell you.)
As valuable as the various points are, however, one rises above the rest. This principle should be tattooed on the forehead of anyone who writes online content. It's point 14: "Use the active voice."
You can distinguish between active voice and passive voice pretty easily. Passive voice uses "be," "am," "is/are" or "was/were." Active voice doesn't. Here are examples of both voices:
"Jack handed the bucket to Jill."
"Jill was handed the bucket by Jack."
Hopefully, you can read or hear the difference. The sentence that uses active voice sounds much better--it's more direct and more concise. The passive voice sentence not only has one more word (remember point 17) but sounds awkward.
I'm not saying you should use the active voice all the time. In fact, you probably can't. Strunk and White themselves write that the passive voice "is frequently convenient and sometimes necessary." I've used it myself throughout this blog. (I just used it in the last heading. Kudos if you caught the irony. Bonus points if you caught the Oscar Wilde allusion.)
However, writers should pay heed: Subtextually, the passive voice says that readers don't need to pay attention to what they're reading. In other words, the passive voice makes for passive readers.
You don't want passive readers, especially if you're writing content for web marketing purposes. You want active readers. You want engaged readers that, hopefully, will convert into engaged customers. More often than not, the passive voice won't get you those readers.
Admittedly, you probably don't need to tattoo "Use the active voice" on your forehead. The world certainly couldn't use any more bad tattoos. Still, remember this: If you want readers to sit up and take notice of what you write, use the active voice.