An unkown space between vertical divs

Recently when I write a HTML code, I suddenly met an unknown space between vertical divs.

The reason that the issue happens is because each browser has different default css. To solve this, you can reset the margin and padding of divs.

* {
    margin: 0px;
    padding: 0px; 
}

The simple code can solve it. Now and then you can customize the divs with your own css.