Using Flex-box in my opinion:
#parent {
display: flex;
justify-content: center;
align-items: center;
}
<div id="parent">
<div id="child">Hello World!</div>
</div>
You see there are only three CSS properties that you have to use to center the child element vertically and horizontally. display: center;
Do the main part by just activating Flex-box display, justify-content: center;
center the child element vertically and align-items: center;
center it horizontally. To see the best result I just add some extra styles :
#parent {
display: flex;
justify-content: center;
align-items: center;
height: 500px;
width: 500px;
background: yellow;
}
#child {
width: 100px;
height: 100px;
background: silver;
}
<div id="parent">
<div id="child">Hello World!</div>
</div>
If you want to learn more about Flex-box you can visit W3Schools, MDN or CSS-Tricks for more information.