There are people who take an effort to produce something and then take the extra step to show it to others so they might learn or collaborate. And those people often get comments where their project is being picked apart with a fine-toothed comb, packaged in an abrasive tone.
This happens over and over again in IT. There's plenty of people with insecurities who - for reasons that psychiatrists understand better than me - rejoice in taking others down.
Lots of people would say it's because we are hidden to each other behind the screen and would never say this stuff to a colleague. I guess that's part of the reason why it's so prevalent in HN. Though, I've seen the same style of rhetoric in the industry, face-to-face.
"You should go to back to school before talking about CSS" and plenty of laughing was something directed at me when I suggested that the ´class´ attribute for HTML tags has semantic meaning beyond just hooking up CSS rules to it. I had trouble explaining it, but my colleague with longer industry experience took it as "lol this noob junior programmer doesn't even understand what ´class´ means". He didn't even stop to consider how class attributes can be used through Javascript and beyond, he immediately went for taking me down as fast as possible.
And that f'ing hurt. And after that I have probably been toxic to other people too. I wish I could go back and undo all the times I have hurt someone like that. Totally unnecessarily and without reason. But I can't. That's why the least I can do when someone suggests that "hey, tone it down a bit" is to shut up with the explanations why I am right to say what I'm saying and think whether there's a more constructive way to go forward.