This works as expected when I run the sample page:
<asp:Panel runat="server" ID="textPanel" BorderColor="Black" BorderWidth="2">
<label>Dock float</label>
<input style="float: right;" type="button" value="PressMe" />
</asp:Panel>
Though, I did find VS designer showing it on the next line as your image, but things were as per expectation when I run the app. Try out.