html - Vertically center a div inside another div

Question

I'm trying to vertical-align: middle a div inside another div, but for some reason it's not working properly. What am I doing wrong?

#wrapper {
border: 1px solid red;
width: 500px;
height: 500px;
}
#block {
border: 1px solid blue;
width: 500px;
height: 250px;
vertical-align: middle;
}

<div id = 'wrapper'> 
<div id = 'block'> I'm Block </div>
<div>

Answer 1

The vertical-align property applies only to inline-level and table-cell elements (source). In your code you're working with block-level elements.

Try this flexbox alternative:

#wrapper {
    border: 1px solid red;
    width: 500px;
    height: 500px;
    display: flex;               /* establish flex container */
    align-items: center;         /* vertically center flex items */
}

#block {
    border: 1px solid blue;
    width: 500px;
    height: 250px;
    /* vertical-align: middle; */
}

<div id='wrapper'>
    <div id='block'> I'm Block </div>
<div>