Wherever there is a byte limit in the database (e.g. topic title, any others?), we should prevent the user from typing too long a post, rather than letting them type too long, then failing it on submit.

Side note, for bonus points we can also tell the user how far they're over (still dynamically, so they can see before clicking submit). StackOverflow (and many text messaging apps) does this. It both shows you how many characters you have spare, and if you go over, how far over you are (They also have a minimum length, so they show how close you are to that, but we don't have that).

There are also color warnings (the remaining character count more red/orange when you get closer to going over).

This should use jquery.bytelength (and if we take the simple approach, jquery.byteLimit jquery.lengthLimit). Unlike maxlength, this is number of bytes, not number of characters.