Allow .btn on [role="button"],[role="link"] with tabindex which ≠ -1

[cvrebert]

Refs #313.

[patrickhlauke]

conceivably, tabindex could also be any valid number >= 0 (but not tabindex="-1") ... not sure that can be fully expressed with CSS selectors though, so perhaps just [tabindex] (knowing that it "may" catch some invalid combos) would be sufficient...

other edge case could be something that acts as a link, but wants to be visually styled as a button (like a call to action that actually opens a new page)...this would have a role="link" (and JS which when ENTER is pressed would the window's location...again, lots of faffing)

[patrickhlauke]

i do wonder if at these stages (where authors explicitly do some very intensive work to use a <span> or similar rather than just a <button> or <a href...>), they may also understand the limitations of the linter...but hey it wouldn't harm either way, as we're just covering edge cases i guess

[cvrebert]

not sure that can be fully expressed with CSS selectors though

No worries, we don't have that limitation. We try to express the constraints using selectors where possible, but we're okay with dropping down to JS when necessary.

[patrickhlauke]

👍