How to stop wordpress from deleting an empty tag without modifying functions php

Yup you enter an empty tag to hold and display something such as a font icon. You build it in the text editor tab of the page and then later switch to the visual tab. Upon saving and reviewing the font icon is not there on the page. You go back to edit and its gone.

Just put an HTML comment inside the empty tag. For example:

<a href="#"><span class="fa fa-arrow"><!-- fontawesome --></span></a>