Google to the rescue—or not!
Here's what one grammatical source said:
Although all forms are seen in written communications, only two forms are correct (on line and on-line); the difference between them (ie the use of the hyphen) is very important and applies to many other grammatical forms.Seems straightforward enough—but can you trust a source that says "ie" instead of "i.e.,"? Plus, it is a site in the U.K. and their usage varies from American usage sometimes...
To say that you are ‘online’ would be like saying that a tennis player is ‘oncourt’ or that a builder is ‘onsite’, rather than saying she/he is ‘on site’. We will never see the back of ‘online’, however, and the overall situation may change. To remain consistent though…
Where the phrase is just a factual statement, we use two words.
Where the entire phrase is used to further describe something else, we must use the hyphen to show this.
How about this: Purdue Online Writing Lab which is the next site Google lists after an ESL site. They don't raise the issue, but just use online as one word in their title. What does that say?
I could cite a bunch of sites (see the pun?), but this one says a lot (two words—not alot!): what is the name of the Merriam-Webster dictionary's Internet presence? Yep, you guessed it: Merriam-Webster Online!
Or, what about the venerable Chicago Manual of Style, my constant companion? Their Internet presence is The Chicago Manual of Style Online!
And my second companion, the The SBL Handbook of Style, what do they say? It might be getting a bit dated, being published in 1999 (before the Internet became the default source for research, what with JSTOR, WorldCat, and others), but even then, in paragraphs 7.3.13 and 7.3.14, they use online as one word...
What's a person to do?
The trials of being a copy editor...the question raised by all this searching? Does one still capitalize Internet? : )