How to select a div element in the code-behind page?

Question

I have a div element:

I want to control this element from the code-behind page an

Answer 1

Give ID and attribute runat='server' as :

//somecode Codebehind:

Access at code behind

    Control Test = Page.FindControl("portlet_tab1");
    Test.Style.Add("display", "none"); 

    or 

    portlet_tab1.Style.Add("display", "none");