Flexbox Playground was done with simple JS, CSS and HTML, it let's you try out flexbox properties and animates the changes.
It was my first big projects that took me about a month, I made it so my flexbox skills will be remembered and other people that have problems with such things will have it easier to understand what does what.
It improved my responsive and flexbox skills by a lot while also finding solutions to some more nuanced problems like animating non-animatable css properties.