Allow HTML tags in TextBox control

I need to be able to insert HTML tags in some TextBox controls in a webpage. I know I can avoid the exceptions of type HttpRequestValidationException by changing the ValidateRequest property of the Page directive to false, as in

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" ValidateRequest="false" Inherits="_Default" %>
But that leaves all TextBoxes on the page open for HTML insertion, which I don't want. I need to be able to insert HTML in only some TextBoxes, not in all of them.
Is there a way to accomplish that?

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