You can on each input field add validation rules. If you specify the type - e.g. here client the validation will be done server side and client side for security reasons.
<JetInput
id="password">
<validate
type="client" name="required"/>
</JetInput>
possible validation types:
required
checked (same as required)
minlength (one argument required)
maxlength (one argument required)
match (one argument, the regex is required)
int
short
byte
creditcard
date
email
float
floatrange (two arguments min and max required)
intrange (two arguments min and max required)
validatewhen (one argument, the id of the related field required)
<JetFieldset
id="password">
<validate
type="client" name="checked">
<arg>
1</arg>
</validate>
</JetFieldset>
possible validation types:
checked (one argument, how many checkboxes or radiobuttons are required)
Special validation messages could be set via the "messageKey" attribute.