Maybe you want to add PayPal, some cool Google map, or vacation pics from flickr but Joomla is being a jerk and stripping out your custom code. Here is what to do:
Switch off the TinyMCE editor and write raw HTML (Site > User Manager > click on the user you want to change > User Editor > select “No WYSIWYG Editor” > Save)
Now you can edit your content without annoying Joomla interfering. After adding your custom content you can turn TinyMCE back on.