function error(reason) {
			future.errorThrows("CustomData with key " + key + " should be written to HTML of " + oRelated + " but " + reason);
			return null;
		}
If set to "true" and the value is of type "string" and the key conforms to the documented restrictions,
this custom data is written to the HTML root element of the control as a "data-*" attribute.
If the key is "abc" and the value is "cde", the HTML will look as follows:

<pre>
  &lt;SomeTag ... data-abc="cde" ... &gt;
</pre>

Thus the application can provide stable attributes by data binding which can be used for styling or
identification purposes.

<b>ATTENTION:</b> use carefully to not create huge attributes or a large number of them.
@since 1.9.0