Responsive using flex-box

Nice work Quan, I always set my width using responsive units like %, then my max-width I use an absolute unit like PX ( so the div can stop expanding when it gets to the max-width). Your width works mainly for your mobile and then when you move to a device with a wider screen your max-width applies (This is just one way to use it, there are other applications of the width/max-width)

check this out https://www.w3schools.com/Css/css_dimension.asp