This avoids applying the reset to named anchors/placeholder links (links without an href) that have explicitly been made keyboard-focusable (using tabindex). This is not fool-proof - it's not easy/straightforward to check for the actual tabindex value itself, to ensure it's positive, not will this apply if a link has been "blessed" with tabindex via JS. However, this should catch most common uses (and gives a reasonably valid way around the issue for developers who, for whatever reason, DO want to use links without href - as side effect, it forces best practice of at least ensuring these links can also be focused with the keyboard)
(Partially) addresses concerns raised in https://github.com/twbs/bootstrap/pull/19411#issuecomment-218106243