@GaryGorsline: Highly interesting. Just for fun I tried the square brackets. Killed even the design screen. Finally found that e.g. w3schools say Hyphens in HTML and CSS:
HTML5 attributes can start with data- (data-quantity, data-price).
CSS uses hyphens in property-names (font-size).
Hyphens can be mistaken as subtraction attempts. Hyphens are not allowed in JavaScript names.
So I just learned something I had not been aware of. Thanks again.