So many of my clients love WordPress but hate the WYSIWYG editor.
The problem:
When users try to add content with HTML within the content - then go back to edit it, save it, and their custom HTML becomes a mess. For most - simply disabling the rich text editor (Users > User > uncheck “Use the visual editor when writing”) is good, but others like to use the editor except for a handful of posts or pages.

The solution:
Download and install WP Codeshield. This plugin will make sure that the rich text editor does not touch any custom HTML code within . All you need to do is wrap around your custom code and you can forget about any more issues with the rich text editor and any HTML you put in/edit.

*Note - if you put this in place and your outputted HTML text looks different than everything else (it is using a monospace font by default) - add : code {font-family:inherit} to your style sheet and the text will not look out of place.