These days most mainstream browsers even have spell checkers built in.
It's just laziness / habit that people continue with their oddities when typing.
Something i must confess to that probably gets on someones nerves somewhere, is the use of acronyms.
TBH, afaik, ofc, etc etc.
In my subjective view it's far more acceptable to write afaik rather than 'i no'.
EDIT:
if you dont know when to use there, their, they're etc then why should i assume you have the intellect of an adult? (dyslexics are excused on this one)
Rightly so, however the number of people that have claimed dyslexia as the reason for their poor spelling is mind boggling.
Dyslexics do get a free pass to a degree, however I'm relatively sure words such as "you" are within their grasp, and so blaming the use of "u" on a condition isn't on in my opinion.
I don't mean to sound harsh, however i know a few dyslexics who strive to improve their written skills daily, often asking me or looking elsewhere for help before making posts on forums (I've sat there and watched one of my friends spend a good 15 minutes rereading a relatively short topic he was posting once to try and get it correct).