Professional writing coach Daphne Gray-Grant says that when writing for the Internet, you should consider the advice public speakers are often given: slow down.
In writing terms, this means focusing on being as easy to read as possible. This is because people read differently at their computers than they do when faced with any printed document.
You may not always be conscious of it but when we’re reading at our computers, a light is shining in our eyes… This is the backlighting from the screen and we usually don’t notice it — except if we’re sitting on a beach in the sunshine and suddenly discover we can’t read at all.
But there are other challenges with computers. The typefaces we like on paper often don’t work on screen. Each letter is represented by square pixels on a grid rather than by lines of ink on paper. This makes them harder to read. As well, our computers have less control over spacing, hyphenation, justification and column width.
Furthermore, the width of a standard column on the Internet is often too wide for the human eye. (When I worked in the newspaper biz, I was always told that you should multiply the point size you used by two to determine what should be the maximum column width in picas. Thus, anything in 9 pt type should be no more than 18 picas wide, or about three inches.) Many Internet sites have columns far wider than three inches!
Bottom line? When you give your readers text on a screen, you’re asking them to work really hard. Thus, it’s more important to “speak slowly” so they understand what you’re trying to say.
She goes on to give a series of specific tips and tactics to make your writing easier to understand online. Many of these tips apply to email as well. Go read Daphne’s post at The Measurement Standard for more details.