In most cases, you don't. Zone identifiers are OS-specific, contain whatever the OS says they do, and may be only valid in the short term (e.g. Linux interface number). You only need them if you are doing lower level networking things. As a web app you just don't, because they aren't part of an internet address. As a web browser you pass whatever the user typed through to the operating system.
Comments
In most cases, you don't. Zone identifiers are OS-specific, contain whatever the OS says they do, and may be only valid in the short term (e.g. Linux interface number). You only need them if you are doing lower level networking things. As a web app you just don't, because they aren't part of an internet address. As a web browser you pass whatever the user typed through to the operating system.