Extract from MSDN :
You are encouraged to read the script to see more of what is going on. However, it is not recommended that you modify these scripts, because their function is very closely tied to a particular version of the run time. If the run time is updated, the scripts may need a corresponding update, and you will have to either lose your changes or face problems with the scripts not working. If you must change the scripts for a particular project, take a copy of the files and point your project to them by overriding the location of the files with a private web.config file.It is perfectly fine to change this location to be a relative or absolute reference.
For more details refer ASP.NET Validation in Depth