Custom Elements are created by extending the HTMLElement class.
Therefore, when you create one, is seems you'd have to take special precautions not to give your Custom Element any custom-property-names that are already inherited from HTMLElement.
I can imagine numerous ways of avoiding these potential naming conflicts, but would be interested in other's suggestions or any best practices on the topic.
As suggested by @Jared Smith, the best practice would be to use hyphenated names, and more precisely the data-* notation, accessible via the dataset property.
Note that the aria-* notation should not be used because it is reserved for accessibility.
In the end you can choose any name, except the global attributes, or the specific attributes if you extend a standard HTML element.