Cannot find answer anywhere: where does the class="DDTextBox" come from?

This is my control:
<aspynamicControl runat="server" DataField="Quantity"
Mode="ReadOnly" CssClass="quantity" />

This is what gets rendered in the browser:
<input name="ListView1$ctrl0$__Quantity$TextBox" type="text" size="5"
id="ListView1___Quantity_TextBox" class="DDTextBox" />

Where does the class="DDTextBox" come from?
I do see the quantity class, but it is on the span element that is the
parent to the input element. Since that gets overridden by the
DDTextBox class in the input element, having it on the span element
does not help.

It must have something to do with Dynamic Data, but I can't find the
answer anywhere. I searched all the file in my solution and DDTextBox
is nowhere to be found.

And where does the size=5 come from? I am able to enter more than 5
characters into the text box, so it seems to have no effect.

By the way, I'm trying to control the width of the input box and right-
align the text in it.

Thanks!