Options to perform Client Side/ Server Side validations on portal forms

Hi,

I have configured Customer Service portal and have some requirements to put client/ server validations and display error message when users submit the web form.

How can these be achieved?

For example,

1)  Show error 'Field not populated' if a particular field is not populated

2) Show error 'No records exists in Sub entity' if the child entity doesn't contains any records.