https://webisfree.com/2018-09-19/css-animation-%EA%B5%AC%ED%98%84%EC%8B%9C-%EB%A7%88%EC%A7%80%EB%A7%89-%EC%83%81%ED%83%9C-%EC%9C%A0%EC%A7%80%ED%95%98%EB%8A%94-%EB%B0%A9%EB%B2%95
from here
