In MariaDB and Percona implementation of the standard is the same as in MySQL
The fundamental issues all the same (the application will not have to change), but some differences still there. This is mainly the improvement of engines, the individual functions and performance optimization.
Full list of differences:
https://mariadb.com/kb/en/mariadb/mariadb-vs-mysql...
Comparison compatibility:
https://mariadb.com/kb/en/mariadb/mariadb-vs-mysql...FIRST (), MERGE, etc. operators are also not supported?
https://mariadb.com/kb/en/mariadb/merge/The job is sometimes written knowledge of MariaDB, Percona so what's the point of "knowledge" is if it is a transparent replacement for MySQL are able to work with it - can with them.
It is unlikely in such vacancies is assumed knowledge fundamentally MariaDB.
Rather, this listing of products that are used nowadays, the employer, and which probably definitely have to work with the applicant, i.e., he expects to use MariaDB not shock You.
May be, of course, that their infrastructure is strongly tied to individual features of MariaDB.
Maybe it's just the attrition of those who fell off the progress, and don't even know what that word means.