Can I use a SQL SAL for a web site or other front end application used by the end user’s employees to interact with SQL Server 2016 Standard Edition?

No, a SQL Server SAL may only be assigned to a user not a device nor any other system that stands between the end users and the SQL Server software. This is not a change to the SQL licensing rules in SPLA today. If you assign a SQL Standard SAL to a user, you must be able to count all users authorized to access SQL Server directly or indirectly. A properly licensed user may access any number of running SQL Server Standard instances.