We use sometimes i tag on Bootstrap themes such as FlexArmor2 for vector icons (e.g. font awesome, GLYPHICONS).
Example of HTML code:
<i class="fa fa-book"></i>
However, because CKEditor removes empty tag, i tag is lost.
You can evade this by adding a lower line to CKEditor4 Other configuration option.
You can deal by adding space.
<i class="fa fa-book"> </i>