Whenever you are in design mode in VS2012 for editing a Web Form, if you click on anything, and accidentally drag and drop, a CSS auto-style will appear at the top of the HTML for the aspx page.
From some preliminary research, I checked the following:
Tools>Options>HTML Designer>CSS Styling>Style Application Mode, but it was already set to manual. Is there somewhere else I should be looking to disable this functionality?
Right now there is no way to turn off this. We have received this feedback through connect link. Please add vote to this connect bug. And we will consider this in future release of VS.