Div and text inside the div behaving weirdly - html

Question

I want to center the text "name" horizontally and vertically inside the div "firstquad". I want the div to have 100% width and 25% height. But the div has much more than 100% width. For the text, I have set the top and left as 50%. The text should be centered and the div should fit the page horizontally but its like this. Any help?
body {
padding: 0%;
margin: 0%;
height: 300%;
width: 100%;
background-color: cornsilk;
}
#firstquad {
position: relative;
width: 100%;
height: 25%;
top: 0%;
text-align: center;
background-color: blue;
}
#name {
position: relative;
top: 50%;
left: 50%;
color: white;
}
<div id="firstquad">
<h1 id="name">ASEF DIAN</h1>
</div>

Answer 1

Both div and h1 are block level elements by themselves.
Block level elements behave in such a way that
they create a line break before and after themselves
they grab as much horizontal space as they can get
Which means that with <div><h1></h1></div> you have a div that grabs as much horizontal space as available (full page width). Inside it, the h1 behaves the same, consuming all horizontal space that the surrounding div allows.
Now with position: relative; left: 50%; you do not change the width of the h1 - you simply change the position, where its rendering starts. Obviously, this leads to the h1 moving partly outside the div. Add borders so you understand:
body { margin: 30px; }
div { border: 2px dotted grey; }
h1 { border: 2px dashed blue; }
<div><h1>Test</h1></div>
Now move the h1 (only slightly, so the effect is visible better):
body { margin: 30px; }
div { border: 2px dotted grey; }
h1 { border: 2px dashed blue; position: relative; left: 20px; }
<div><h1>Test</h1></div>

Answer 2

css:
body {
padding: 0%;
margin: 0%;
height: 300%;
width: 100%;
background-color: cornsilk;
}
#firstquad {
position: relative;
width: 100%;
height: 25%;
top: 0%;
background-color: blue;
display: flex;
justify-content: center;
align-items: center;
}
#name {
color: white;
}

Answer 3

body {
margin: 0%;
height: 100%;
background-color: cornsilk;
}
#firstquad {
height: 25%;
background-color: blue;
text-align: center;
}
#name {
color: white;
margin: 0;
}
<div id="firstquad">
<h1 id="name">ASEF DIAN</h1>
</div>

Answer 4

Is this what you are looking for?
Just change #name to #name {
color: white;
}
body {
padding: 0%;
margin: 0%;
height: 300%;
width: 100%;
background-color: cornsilk;
}
#firstquad {
position: relative;
width: 100%;
height: 50%;
text-align: center;
background-color: blue !important;
}
#name {
color: white;
}
<div id="firstquad">
<h1 id="name">ASEF DIAN</h1>
</div>