Amy Gahran at Contentious suggests that proper grammar and punctuation on the web is evolving:

These considerations can help guide grammar and punctuation choices in your online writing:

It's not print. Most formal rules of English grammar and punctuation were developed to suit written (printed) communication, and they still work very well in that environment. However, print is only one medium -- and in coming decades it may cease to be the most common communication channel in many geographic regions or sectors of society.

It's a challenging visual environment. Text and images (both visual vehicles) are the primary ways to transmit messages via computer. Unfortunately, today's computer screens remain a more difficult physical environment for reading, thanks to lower resolution, flicker, lighting, etc.

Small punctuation gets lost. Look at your keyboard -- the most commonly used English punctuation marks are small. In print, punctuation marks serve to enhance the perceived flow of words. However, on a computer screen commas, periods, semicolons, colons, apostrophes, and many other common punctuation marks are simply hard to see. Therefore, less punctuation and bigger punctuation marks are usually more effective in online content. This is why the em-dash (a long hyphen: --) tends to be used more liberally online than in print. SimSimilarly, semicolons (;) tend to be used sparingly in web content – they’re too visually innocuous to play the key structural role that they often do in print.

...via elearningpost