Thanks for that.
I did put in a Height of 800px and that seemed to resolve it, I don't know if it worked across all browsers.
But I did try what you suggested removing the 'height' and found in 'reduce down' mode, there was a white line below the bottom of the screen.
So I kept what you suggested and added the 'height' again at 800px
As I say, it seems to be OK, it not a commercial site, just something for my family, so not hugely important, but it bugs me when I don't understand something and can't work it out.
We don't use absolute positioning for vertical centering anymore. With CSS Grids, there's less code and none of the consequences that come from positioning elements absolutely.

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>CSS Grids - vertical centering</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<style>
body, html {
margin: 0;
height: 100%;
display: grid;
background: #333 url(https://placeimg.com/1200/900/nature) no-repeat center center;
background-size: cover;
}
main {
padding: 2%;
width: 80%;
text-align: center;
margin: auto;
background-color: rgba(0,0,0,0.5);
color: #FFF;
}
</style>
</head>
<body>
<main>
<h3>Welcome to CSS Grids!</h3>
<p>I'm vertically & horizontally centered.</p>
</main>
</body>
</html>
Give it a try 🙂