Implement horizontal center positioning of div

Question

I want to position a div in the center of the screen. The problem is that when the div is small, 45% from the left looks fine, but when the div is longer (i.e. wider in width), I need to change it to 30% from the left.

Is there a smart way to position a div in the center based on its size.

body {
background:blue;
}

.box {
position: absolute;
top:10px;
left:30%;
background:white;
padding:10px;
border-radius:10px;
}

<div class="box">
This is a long div, so left = 30% is needed
</div>

Answer 1

You can center it horizontally by:

.box  {
   position: absolute;
   top:10px;
   left:50%;
   background:white;
   padding:10px;
   border-radius:10px;
   transform: translateX(-50%);
}

Answer 2

You can use the display: flex;property to center align the component.

css flex

body {
flex: 1;
background:blue;
display: flex;
justify-content: center;
}

.box  {
position: absolute;
top:10px;
background:white;
padding:10px;
border-radius:10px;
}

<div class="box">
这是一个很长的div，所以需要左边=30％
</div>