IPv6 address validation and canonicalization
On POSIX systems you can use inet_pton and inet_ntop in combination to do canonicalization.  You will still have to do your own CIDR parsing.  Fortunately, I believe the only valid CIDR syntax for IPv6 is the /number_of_bits notation, so that's fairly easy.
The other issue you will run into is the lack of support for interface specifications.  For link-local addresses, you will see things like %eth0 on the end to specify what link they are local too.  getaddrinfo will parse that but inet_pton won't.
One strategy you could go for is using getaddrinfo to parse and inet_ntop to canonicalize.
getaddrinfo is available for Windows.  inet_pton and inet_ntop aren't.  Fortunately, it isn't too hard to write code to produce a canonical form IPv6 address.  It will require two passes though because the rule for 0 compression is the biggest string of 0s that occurs first.  Also IPv4 form (i.e. ::127.0.0.1) is only used for ::IPv4 or ::ffff:IPv4.
I have no Windows machine to test with, but from the documentation it appears that Python on Windows supports inet_pton and inet_ntop in its socket module.
Writing your own routine for producing a canonical form might not be a bad idea, since even if your canonical form isn't the same as everybody else's, as long as it's valid other people can parse it.  But I would under no circumstances write a routine of your own to parse IPv6 addresses.
My advice above is good for Python, C, and C++.  I know little or nothing about how to solve this problem in Java or Javascript.
EDIT: I have been examining getaddrinfo and its counterpart, getnameinfo.  These are in almost all ways better than inet_pton and inet_ntop.  They are thread safe, and you can pass them options (AI_NUMERICHOST in getaddrinfo's case, and NI_NUMERCHOST in getnameinfo's case) to keep them from doing any kind of DNS queries.  Their interface is a little complex and reminds me of an ugly Windows interface in some respects, but it's fairly easy to figure out what options to pass to get what you want.  I heartily recommend them both.
What libs have you used for that?  How compatible are they with one another?  Or did you write your own parsing routine?
I'm particularly interested in mutually-compatible implementations for Java, C++, Python, and JavaScript, which support:

zero compression ("::")
IPv4-mapped addresses ("::ffff:123.45.67.89")
canonicalization (including to the short form, for human readability)
CIDR-style netmasks (like "/64" at the end)