Steve Kraus wrote:

Someday I fully expect the term "it's" to
become fully acceptable in standard English in the possessive sense and
then the barbarians will have won. In the meantime I wish web forum
software would come with a plug-in that would, on displaying posted text,
randomly convert "it's" into "it is". Correct users will still be correct
if a bit stilted. Incorrect users will be made to look moronic.

That is not always true. For example:

It's come to my attention that. . .

In this case "It's" means "It has"
--
I don't understand why they make gourmet cat foods. I have
known many cats in my life and none of them were gourmets.
They were all gourmands!