You want to disable auto validation of HTML elements.
The blogengine who uses Tiny_mce editor validates HTML elements and strip out
non HTML code from it. It gives problem while enabling tracking scripts
or configurin advertisers AD. This feature really helps you to avoid error tags
in HTML.
In below image you can clearly see the non HTML
attributes. When you say update, it get vanished. Lets see how we can disable this feature.

Steps:

1. Open tiny_mce.js
   Remember its production side scripts so keep a backup of it. Also note that the tiny_mce_src.js
   which is non compressed version of tiny_mce.js.
2. Find element named verify_html.
3. Set verify_html:false
4. Save the file.
5. Upload and replace the production file with yours.
6. Delete temporarily internet files from your browser or clear cached of it.
   This ensures that you will get newer version of tiny_mce.js.
7. Browse the updated web and try to edit the content and add HTML and say update.
8. Now, your problem is resolved, if you still have any question contact to tiny_mce forum on web.

There are so many open source HTML editors also know as WYSIWYG (What You See Is What You Get).
Tiny_MCE and FCKeditor are widely used for asp.net, PHP and Java websites.