Why do I cannot center elements if i scale them - html

Question

I am using this to center things in CSS:
.testclass {
display: flex;
justify-content: center;
}
but when i want to scale elements using width and height, it doesn't work and my elements are not centered.
Like this:
.testclass {
display: flex;
justify-content: center;
width: 200px;
height: 200px;
}
What's the problem?

Answer 1

This looks like the expected behavior.
Remember that in this case justify-content: center; centers what is inside the container - not the container itself.
EDIT:
I added margin: 0 auto; to center the container.
#container1 {
display: flex;
justify-content: center;
border: 1px solid red;
}
#container1 > div {
border: 1px solid blue;
background: yellow;
}
#container2 {
display: flex;
justify-content: center;
border: 1px solid red;
width: 200px;
height: 200px;
margin: 0 auto;
}
#container2 > div {
border: 1px solid blue;
background: yellow;
}
<div id="container1">
<div>test 1</div>
</div>
<div id="container2">
<div>test 2</div>
</div>

Answer 2

display: flex; and justify-content: center;
works for parent elements. That is, child elements of that particular parent will be centered, not the parent.
To center .testclassHTML
<div class="parent">
<div class="testclass"></div>
</div>
CSS
.parent {
background-color: red;
display: flex;
justify-content: center;
}
.testclass {
background-color: green;
width: 200px;
height: 200px;
}

Answer 3

If you want full center (horizontal vertical) you can use this code:
.container {
position: relative;
width: 100%;
height: 100vh;
display: flex;
justify-content: center;
align-items: center;
}
<div class="container">
<div class="testclass">Content</div>
</div>