I just spent about 3 hours reading posts about this problem. They go back as far as 2006 or sooner. It appears that if you add HTML in the Text mode and then go to the Visual mode, The Editor will delete the code if it doesn't know how to render it. All these updates and all these years would suggest someone has fixed this problem. The only solution offered was pick HTML and stay with that. If we were going to stay with HTML code, why would we use WordPress in the first place? There has to be a way to encase code so it's not affected by the editor. Has anyone figured this one out? I have yet to find any new posts about this problem.