How <html:javascript> works in Struts 1.3?

Hi,

I am new to struts, while i am referring in some of the website, i got a doubt in the concept of enabling client level validations using struts validator framework. We are having all the  validations in the validator-rules.xml, And required validations for the form fields in validation.xml, (So all are in server side only).

According to the website information which i referred, validation will perform only at client side by using <html:javascript> tag, So our data will be validated at client level itself, so there is no communication required.

Here my doubt is, if there is no communication is required to server, then how the client will know for what validations has to perform for which fields, because all these info in validation.xml(server side only).

At lease one time communication is required right?

Please clarify...