Joomla has an in built contact form validation. But Joomla does not provide an error message or popups for the invalid entry. so sometimes we don't notice the warnings because of a mismatch in the css. In order to solve try adding the following line in the template.css.
.invalid {color:red;}
Now the labels will be shown as red color if invalid data is entered, and also it will not allow form to submit.
