How to hide from UI, but it should still work?

Hi,
I'm working on adding new UI design to an existing web forms application. On 1 .aspx page, there are some input fields, and for each field, there is <asp:CustomValidator...>
The old design put the <asp:Textbox > field in 1 table column, and the <asp:CustomValidator> in another column; and the error message only shows up when there is an error.
Now, the new design completely removed the table, and replace it with <div>; which creates an issue - if I put <asp:CustomValidator...> right below the <asp:Textbox...>, then it adds more space below the <input> field generated by <asp:Textbox...>; and it does not work with the new UI.
So, I tried to hide the <asp:CustomValidator...> when the page first loads; this solved the extra spacing issue, but when I type in something incorrect in the <input> field, it does not call the validator at all - so the <asp:CustomValidator...> is not being ...