With you’re using RadEditor Control in SharePoint you can customize almost everything. The problem is that the special HTML Tags are normally ignored by SharePoint for security reason.
Telerik tried to solve this problem on putting a Base 64 converted string in your field once you posted the modifications. Your field, so, will be something like this:
<pre style="display:none" id="RadEditorEncodedTag">PGlmcmFtZSBzcmM9Imh0dHA6Ly93d3cuZ29vZ2xlLmNvbSI+PC9pZnJhbWU+</pre>
If you want to use your field you will face so to some problem. To solve it you have to use the static method:
With this method your field will be parsed and the real html will return from this method.Read More