Why 1.3

Targeting NetStandard1.3

We target NetStandard1.3 because of the following are not available in < 1.3:

System.Diagnostics.Process: This is used in Google.Apis.Auth.OAuth2.PromptCodeReceiver to launch a browser as part of the interactive oauth2 flow.
System.Security.Cryptography.Csp: Provides various RSA cryptography functionality as required by Google.Apis.Auth.OAuth2.ServiceAccountCredential. E.g. System.Security.Cryptography.RSACryptoServiceProvider.
System.Reflection.TypeExtensions: Provides compatibility extension methods for 'old reflection' source code, to 'new reflection' calls.
System.Net.Sockets: Currently used in MediaDownloaderTest.cs to find an available port.

And the following are not available < 1.1:

We could probably target NetStandard1.1 without too much problem.

System.Diagnostics.Process: Remove support for launching a browser, simply display the URL to visit using Console.WriteLine().
System.Security.Cryptography.Csp: It is probable that Portable.BouncyCastle provides all the RSA cryptographic functionally that we require.
System.Reflection.TypeExtensions: Providing our own compatibility extension methods is fairly simple. It'll be 100 or so lines of code.
System.Net.Sockets: The available-port finding in MediaDownloaderTest.cs should be easily refactorable.

These changes can be made post-release. Moving from targeting 1.3 to targeting 1.1 will be a non-breaking change.

This is unrealistic due to the extensive use of System.Net.Http throughout these support libraries.