When editing a node, some markup disappears
I have the same issue, and this is caused by the WYSIWYG editor: When you try to edit a node created with a input format with WYSIWYG enabled, the HTML markup will be changed by the editor.
To solve it, you have two options.

Disable the WYSIWYG editor by default, unselecting the checkbox as shown in the screenshot.




Create a node using an input format that doesn't use the WYSIWYG editor (for example Full HTML, or Filtered HTML). If the Full HTML format is already associated with a WYSIWYG profile, try to create a custom format from /admin/config/content/formats, and use it to create new nodes. Remember to not associate a WYSIWYG profile.