I am a massive hypocrite as my understanding of punctuation and grammar rules is abysmal for someone who frequently gets paid to write (write, not edit!).
I am also a very lazy writer on forums, and spend a large part of most days writing very conversationally to clients via a real time messenger portal, so I will forget to capitalise 'I' and I'll use American spellings and slang (particularly when actually talking to people in the USA).
I also type very quickly and often my fingers will put in the wrong word without my brains knowledge, so I might think 'cue' and write 'queue' or vice versa!
Language always evolves and we're likely to see what I think of as a slide back to earlier times when spelling was widely considered optional and sentence structure rather different!
What really pisses me off though, is people who are not dyslexic, who get very riled at others not understanding what they say, but won't do a thing to improve their written communications. They're almost always asking for help with something, often feel as if someone owes them that help, but will not make it easier for that person to understand what it is they need!
It is pure laziness when someone writes 'Ano' instead of 'I know', or to not break up sentences with commas and full stops, or use paragraphs.
The other thing that rather irrationally gets on my wick is 'sth' or 'smth'. Abbreviating phrases, or several words, such as BRB or shortening words like 'identification' to ID is fine, but 'sth' or 'smth' for 'something' truly grinds my gears!
I tend to sit on my hands when people use the wrong word or incorrect spelling, particularly if it is just once. If they repeatedly use the wrong word, where that word has a totally different meaning to the one they intended or in fact the opposite meaning, then depending on context, I might say something.
Eg. 'defiantly' vs 'definitely' - being definite vs defiant are two very different things!
There is someone I know who uses 'thew' instead of 'few' which frankly, baffles me and I think it's related to his accent as well as dyslexia!