After that how does it work if URI used in taglib directive does not map any entries?
Then it will fail unless, you will have the fallback scenario:
From JSP specs The fallback interpretation is targetted to a casual use of the mechanism, as in the development cycle of theWeb Application; in that case the URI is interpreted as a direct path to the TLD
the fallback rule allows a taglib directive to refer directly to the TLD. This arrangement is very convenient for quick development at the expense of less flexibility and accountability. For example: