Have you ever read a story with her's, your's, their's and doubted your grammar skills? I have come across several stories using possessive pronouns incorrectly. I know there are some non-English speakers (and some English speakers) that have used and are using possessive pronouns incorrectly, but hopefully, some is kind enough to correct them.
When using yours, theirs, ours, hers, etc., there is no need for an apostrophe because the words imply that someone owns the object in question.
For example,
That book is hers not yours. There is no need for an apostrophe because the words hers and yours tells the readers who owns the book.
If you want to use an apostrophe, use it when directly acknowledging that the person owns an item.
For example:
That book is Karen's. In this case the apostrophe is need because the own is identified correctly.