I want to be able to call out cases where a custom tag has been used in a context where it's just not applicable.  <tr> outside a <table> is a traditional HTML example of this scenario, but we don't seem to have any allowances for that scenario in the custom elements spec.

"attachedCallback
Unless specified otherwise, this callback must be enqueued whenever custom element is inserted into a document and this document has a browsing context.
"

I was thinking that the component author should be able to raise an exception from the attachedCallback to indicate that it's a wrong context for creating the element.  Not that we can't console.log() to let component-users know about the problem, but so that the user-agent can have the indication that the element is invalid, perhaps triggering a visual treatment for the tag in developer tools.