When any runtime error occurs while generating the element, the element will be replaced with result of expression in the tal:on-error attribute.

Runtime errors are references to non-existent variables/paths/macros and any PHP exceptions thrown in the tag content.

            <span tal:on-error="string:No username defined here" tal:content="user/name">the user name here</span>

          

If an error occurs accessing ' name' or ' user', the error string will be shown instead of the <span> element.

This also works on more than one level of template:

            <span tal:on-error="string:error occurred somewhere"><span tal:content="user/firstname"/><span tal:content="user/lastname"/><span metal:use-macro="userMenu" /></span>