Why isn't the content for the following iframe code showing within a post?

(Actual EventID has been replaced with x's. But src='http://www.google.com' doesn't work either.)

<div style='width:100%; text-align:left;'>
<iframe width='100%' scrolling='auto' height='230' frameborder='0' marginheight='5' marginwidth='5' src='http://www.eventzilla.net/web/event_ext.aspx?EventID=xxxxxxxxxx' ></iframe>
</div>

Solution:
Global settings/Plugin settings/Custom TinyMCE init:

theme_advanced_toolbar_location : "top", height : "400", extended_valid_elements : "iframe[id|width|scrolling|height|frameborder|allowtransparency|hspace|vspace|marginheight|marginwidth|align|src|name]",

HOWEVER, I still had to set 'XHTML validation' & 'XHTML validation on XML-RPC calls' to Basic in order for the iframe tags to be allowed.

Why?