<span petal:on-error="string:booo">$this/aint/gonna/work</span>
<span petal:on-error="<<">$this/aint/gonna/work</span>
<span petal:on-error="string:booo">$this/aint/gonna/work</span>
<span petal:on-error="<<">$this/aint/gonna/work</span>