HTML Code Keeps Disappearing

So, for whatever reason, some of my HTML code continually disappears. I’ll save it once, and if I go back to edit that page, some of my formatting is just gone and I have to go back and edit it again. It’s been a problem for as long as I can remember and now I just want to get rid of it.

This happens especially with my <font> tags and header tags.

This has nothing to do with my theme, as I’ve experienced it on other themes. I also use Visual Composer, and it seems that when I use the frontend editor, none of this happens. However, when I use the backend and save, the formatting disappears.