Sorry, you've lost me.
How can the page appearance change when you simply copy the source HTML to a file and then display that file???? HTML is HTML is HTML. The browser has no idea whether the HTML is coming from an ".html" page, an ".aspx", an ".asp" page, a ".jsp" page, a ".php" page, etc.
If, perhaps, when you copy the HTML and try to show it in the browser, you are not getting all the ".
js" files or ".css" files that are used in the ".aspx" page, then it would make sense, of course. So try to make the .html version use all the same other external files, to make the experience completely the same.
The code you show makes me immediately suspicious that this is exactly where the problem lies, because you are changing the visibilty state of 3 separate things when the button is clicked:
imgbtn_cal1.Visible = False
txt_StartDate.Visible = False
cal_start.Visible = True
If the styles of those three things are similarly separated, then wouldn't you need to coordinate there style changes, as well???