-observed its-it's error is due to confusion over a specific exception-rule of prescriptive grammar
. In English, possessives are usually formed with 's
. However, this rule does not apply to pronoun
I, me → my, mine
you → your, yours
he, him → his
she, her → her, hers
it → its
we,us → our, ours
they/them → their, theirs
In English, we also use 's
to form a contraction with "is", as in "Bob's here." Note that "Bob" is the subject of the sentence — we normally don't see constructions
like that you killed Kenny's cool. ("The fact that you killed Kenny is cool.")
The possessive pronouns are either novel or are built on the object case -- with the exception of it → its.
Nobody would say "Her is here", so there is no confusion over whether "Her's here" or "Hers here" is correct — the question doesn't arise.