I pride Webflow for being an empty canvas that allows me to create custom web experiences without writing code. I believe that Default Styles contradict that premise. As I inevitably need to override them using additional CSS, they also make me worry about how clean my code really ends up being. I would encourage Webflow to git rid of Default Styles and fully embrace their original premise of being a clean, visual way of building for the web.
I too, thought this product was an empty canvas. I just spent 2 hours trying to figure out why my navbar items did not behave according to my flexbox CSS. I eventually found out that Webflow adds floats(!) and pseudo elements on their navbar and navbar-items or whatever it is. Thanks for wasting my time, Webflow. Say hi to 2012 for me – I miss it, too, but not for the CSS.