Automatisch Anpassen

Ich hatte mir @media durchgelesen aber verstehe das nicht.
Kommentar abgeben zu diesen Beitrag/Code ?
Dann hier klicken

Der hier verwendete Code

<style>
a{
display:none;
margin-left:45%;
font-size:50px;
}
@media (min-width: 1px) and (max-width: 400px){
div{
background:red;
width:100px;
height:100px;
}
}
@media (min-width: 401px) and (max-width: 600px){
div{
background:green;
width:200px;
height:100px;
}
a{
display:block;
}
}
@media (min-width: 601px) and (max-width: 800px){
div{
background:blue;
width:200px;
height:200px;
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