The link provideded by Mr.Somkiat is written by our sheriff, Mr.Thomas Paul...
U can have a look more about the client-side validation of Struts at
Sample Chapter-6 of the "Struts, The Complete Reference" by Mr.James Holmes... I do hope that chapter will provide you more info on client-side validations... Hope it helps...
Co-author of SCMAD Exam Guide, Author of JMADPlus
SCJP1.2, CCNA, SCWCD1.4, SCBCD1.3, SCMAD1.0, SCJA1.0, SCJP6.0