Hi I'm new to forum so hopefully posting to correct category .

I just need help with my issue with css div . If I add 1px of padding or 1px border it add at least 10px on div height .
Concrete example here

HTML
<body>
<div id="mydiv">
<h1>Welcome!</h1>
</div>
</body>
CSS
#mydiv
{
padding-top:0px;
background-color:red;
border-radius:0px;
width:400px;
margin:auto;
border: 0px solid black;
}
Thanks for reply