• You can do, and say, nothing. Actually, most of us are taught that this is the polite thing. It's really none of your business, you think to yourself. No skin off of my teeth! Why should I bother? And this response is both right and wrong, when you think about it. Being seen as rude is something very few people actually want, so they say nothing. This reluctance to criticize someone else really depends on the urgency. If I see someone driving around with a flat tire, I'll say something. No, I'm not criticizing their car, or their driving, I know that they could potentially be hurt, or killed, if it wasn't brought to their attention. If I don't like the bumper stickers on their car, I should probably shut up.
• You can say something in a rude way. This is surprisingly easy, even if you really don't mean to be rude. Sometimes it's just the abruptness of it that's rude. Of course, if someone's car is on fire, there is no need for chit-chat. If you've enjoyed an article on the web, and saw that they wrote there instead of their, you may want to preface your comment with how much you enjoyed the article.
|A cute dachshund|
As a Graphic Design teacher, I have been paid to critique. That's the word that art teachers use when they are telling a student what they have done wrong. Believe me, it's miserable, and I never really liked it, but it was part of the job. Nowadays, I don't avoid criticizing people for fear of being rude, I avoid it as I'm just too lazy, and besides, I'm not getting paid to critique, unless it's in a classroom.
If you want to receive corrections, ask for them. Thank people who do it for you. Give praise, don't be defensive. Speaking for myself, I just love to hear from experts who will kindly point me in the right direction. These people are precious to me!
So, there you go. The web is a great place to find things that are wrong, that you can correct, which is true of the world overall. If you do it right, you make the world a better place, if you do it wrong, well, everyone will just kind'a hope that you wander off. Do it right, and you will be welcomed warmly, especially by me!