Repetition is the point

Practice

Three of these reward doing them badly at first. The subnetting trainer in particular needs twenty minutes a night for a week, not three hours the day before.

The cards come back on their own. A card you get right returns in a few days, then a week, then three. A card you miss comes back tomorrow, and again before you leave. So a sitting is short and it ends: the due count goes to zero and the page says so. New cards are never pushed at you, you start eight when you want them. Miss a week and nothing is lost or broken, there are simply more cards waiting. None of it leaves your browser.