HTML5 Conformance Checker: onforminput not allowed where it should be

Hi there,
I'm just starting with HTML5.
On page   http://www.w3.org/wiki/HTML/Elements/output
it is indirectly stated (by the given example) that the onforminput event handler is allowed within <output>.
Confirmed when using the "See also" link which links to   http://www.w3.org/wiki/HTML/Attributes/_Global
However, trying to use this example the validator returns an error.

Thanks and regards
            Volker


example code:
<form onsubmit="return false">
  <input name="a" type="number" step="any"> +
  <input name="b" type="number" step="any"> =
  <output onforminput="value = a.valueAsNumber + b.valueAsNumber"></output>
</form>
Error
*  [Error] Line 95, Column 66: Attribute onforminput not allowed on element output at this point.

  <output onforminput="value = a.valueAsNumber + b.valueAsNumber"></output>
Attributes for element output<http://www.w3.org/html/wg/drafts/html/master/single-page.html#the-output-element>:
Global attributes<http://www.w3.org/html/wg/drafts/html/master/single-page.html#global-attributes>
for<http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-output-for>
form<http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-fae-form>
name<http://www.w3.org/html/wg/drafts/html/master/single-page.html#attr-fe-name>




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