I'm not as impressed, tried to use my Spanish digital certificate and got "the submitted certificate is not an electronic DNI".

Is it this thing: https://op.europa.eu/en/publication-detail/-/publication/f14... (scroll down to Spain)?

No, it is not. In Spain, national and regional administrations support different methods for digital identification by their citizens. One is the certificate embedded into the chip in the national id card, arguably in everybody’s pocket, but, effectively so cumbersome to extract that virtually no one makes use of. That's the one this website claims to supports. Others methods are certificates issued by The Ministry of Treasury among others, downloadable after in-person identification, so used by far more people than the former, but, then again, complicated to use on mobile devices, and lastly specialized official TOTP apps which issue temporary codes for an authentication service that government web pages plug into. The latter, not supported by this webpage, is, I assume, by far the one used most commonly.

Mhm, that's interesting. And quite unfortunate. :\