Sometimes I wonder if anybody even bothers to proofread any more, online or offline. Proofreading takes extra time, yes. Proofreading requires thought, yes. Proofreading is boring, yes. But proofreading is good!
Blogs seem to be especially bad with this, since they already carry a "this just came from my head, seriously" type of essence. But they don't need to sound like that all of the time. This especially goes to people who manage public blogs. Shame! It's not enough to write a post, run the spellchecker, and then click "Publish." Their are many things that due knot got fixed unless yew seas them when you've proofreading!
Spelling errors and grammar mistakes usually make up the bulk of these very easy-to-catch items, but the ones that really irk me are the ones that are obviously side-effects of an edit. The incorrect tense that went with a different phrasing. The incomplete word that used to begin or end a sentence. The phrase that is repeated at least twice, maybe more, but written differently each time. It's obvious to me that you took the time to re-edit the sentence. But did you read it after you "fixed" it?
I'm not talking about small errors, like easy typos, where one letter is replqced by another, or where the order of charcaters gets a little scrambled. I'm talking about obvious mistakes that you can catch just by doing one read-through. When you publish something that is obviously not proofread, it says to the reader, "I didn't read my own article, why should you?"
And if you're going to argue that "replqced" and "charcaters" are obvious, well, you were looking for them, weren't you?