Dos of Web Writing
Do write simple, smart copy
Most people don't read on the Web-they scan. Reading on the Web is more difficult than reading a book or magazine (25% slower) so do your best not to be long-winded or overly descriptive. Get to the point, but do it with style. It's wise to keep your writing at an 8th grade level (or lower) as well. You can check this through the editing tools in Word. When it comes to Web writing, simple is smart.
Do Think Like a User
Try and put yourself in the users place. Don't focus so much on your company, blog or brand-focus on what you have to offer the reader. It sounds like a small tweak but if you do this right your readers will notice. This also has to do with tone. Make sure and decide what kind of tone you're aiming for and stick with it throughout your writing.
Do Prioritize Content
Make sure and emphasize the right content and organize your copy in a clear, logical order. Use headings and sub headings for better scanning. Break up long paragraphs, use bullet points for lists and make sure that you chunk your copy from most important to least. In this digital scanning age you don't get a lot of chances to connect with readers. Make sure your best is at the top.
Do Use Standard Web Practices
Keep your fonts simple and standard. Avoid fancy or cute letters in your copy. First, they're hard to read; second, most users won't see them anyway unless they have the specialized font loaded on their system. Also, when using hyperlinks keep to the standard blue and underlined.
Do Proofread Your Copy
This is a simple but critical step. Don't overlook the importance of reviewing (or having someone else review) your writing before you hit publish.
Don'ts of Web Writing
Don't Bury Your Lead
Write good headlines/headings to keep your readers interest. Do this by front loading your headlines with a good subject + verb structure. Don't bury the subject at the end of the headline.  Use this principle for the headings on your blog, announcements and email subject lines. Now that I think of it-especially with email subject lines.
Don't Use Insider Jargon
This is hard to avoid because we love our "insider" words and phrases-they're like second nature to most of us. However, these words are often distracting and unhelpful to the user. Use simple words and define any difficult terms. Insider language is an easy way to alienate users. This isn't dumbing down your content-it's making it clear.
Don't Overuse Bold Fonts or All Caps
In studies people often pass over bulk sections of bolded words or all caps. Bold works with short headlines but not lengthy sections of copy. Keep it easy on the eyes and avoid chunks of heavy bold or caps in your copy.
Don't Use Long Blocks of Text
Break up your text into short paragraphs or bullets and lists when possible. Avoid long stretches of copy that slow down the reader. Users need more pauses and breaks when reading Web copy, do your best to make your copy crisp, clean and with quick breaks for the reader to scan quickly.
Don't Get Crazy with Your Nav
This last point has more to do with usability but it's important to use standard practices for navigation to help your readers find what they're looking for on your site. The nav is a tool-don't break it by making it hard to read or placing it in a weird location.