If you have five spare minutes, Faces Plus Pairs will fill them. Everything happens on one screen with a 6×6 grid with 18 pairs, and the whole thing runs straight in the browser - no downloads, no sign-ups, no fuss.
Two cards flip per move; match them and they stay open, miss and they flip back after a beat. The whole skill is building a mental map of what you have seen without losing your place. With a 6×6 grid with 18 pairs, later flips are genuinely tense - you know the pair is out there somewhere.
Nothing here needs a tutorial or a wiki. Controls are one gesture, the goal is one sentence, and the difficulty comes from execution rather than homework. That is exactly what a browser game should be.