I spent a week building a mobile-friendly flash card site. I'd like to introduce: Flashmarked!
Flashmarked.com makes converting markdown to flashcards easy. Simply copy and paste your markdown and it'll create cards for each 2nd level and above headers (and the contents of each header will be the back of the card.)
I've done my best to make it mobile-friendly by including swipe controls and also hiding everything but the card when flipping to landscape.
Tech stack is Vue.js + Firebase and pure.css. There's a very basic scoring system in place (using a priority queue) that remains to be fine tuned. Users score cards by double tapping... the further right the more "correct" or "known" the card is... the further left the more "incorrect" or "unknown".