It's not really deprecated, but it's so old and cumbersome and unsupported that the MDN page says "it is better to continue to consider this feature as deprecat

It's not really deprecated, but it's so old and cumbersome and unsupported that the MDN page says "it is better to continue to consider this feature as deprecated and going away." Chrome promises to drop it entirely in version 54, and Firefox is okay with removing it too. You're already warned in your developer console if you use the element. I would love to see a revitalized version of this element, because client side authentication by way of certificates is really cool. It's interesting in comparison to the typical username/password auth ubiquitous today.