To be able to declare binding of Custom Data in your XML view you need to use a special name space, something like that:
and then you can use it in a control declarations:
<Button text="My Button" custom:key="Very special key" />
Wonders begin when you omit name space declaration (for example, by copy-pasting a control declaration from one fragment to another).
In that case Web IDE would never report any problem, everything will be built, no errors will be shown in the browser console, however the control with declared custom data won't show up.
Is it a feature?