When is it grammatically correct to create a contraction of words followed by “is”?

Question

When is it grammatically correct to create a contraction of words followed by "is"? For instance is "who's" a correct short form of "who is"?

Answer 1

The word "is" can always be contracted, provided it is not stressed - though this is considered somewhat informal.

So, in informal contexts,

Fred is taller than Jim.

can be shortened to

Fred's taller than Jim.

...it doesn't depend on what word comes before "is". But if the is is emphatic:

Fred is taller than Jim.

then it would be wrong to contract that to

* Fred's taller than Jim.

That is probably fairly obvious - if you are stressing a particular word, it doesn't pay to shorten it. But there are also other situations where the is has the emphasis, for instance

I don't know what it is.

cannot be contracted to

* I don't know what it's.

even though

I don't know what it is doing.

can be reduced to

I don't know what it's doing.

Answer 2

Who's is the correct contraction of who is, in the same way let's is the contraction of let us.

There are no grammatical rules about not using 's as a contraction of is.
In some cases, there could be a confusion between 's used as contraction of is, and 's used as possessive, but the rest of the sentence should clarify which is the exact meaning of the contraction.

Geraldine's back
Barak's thinking