This message is displayed if you right-click a control on an ASP or HTML page and then choose Convert to Run-time Text.
If you want to use the built-in functionality of the control, do not convert it to run-time text. If you want to script the control yourself (and you are certain of your ability to do so), you can go ahead and convert the control.
Note that you can view the complete text of a control temporarily using the Text View command. However, don't add a new control or modify an existing control while you're in this view. This will cause a loss of functionality. Design-time controls must be added and modified in Graphical view.
For more information on scripting design-time controls, see The Scripting Object Model.