I always have trouble spelling, or more likely, getting the letters in the right order. I can't look up a word in the dictionary if I can't spell it. I could use a spell checker that looks at the first two letters and suggests spellings.
It's been awhile since I used a web browser that did not provide an integrated spell-checker. So I would say, increasingly, that spell-checking has become a client-side issue rather than something that every website should have to provide.
It would simply be a waste of the development team's resources.
I wrote that over three years ago.
Such a basic tool should be available through your browser. If it isn't, it's time to switch browsers or upgrade. There's no value to having each website spend resources providing that is already available (and much more functional) elsewhere.