The alert box is a system object, and not subject to CSS. To do this style of thing you would need to create an HTML element and mimic the alert() functionality. The jQuery UI Dialogue does a lot of the work for you, working basically as I have described: Link.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>jQuery UI Dialog - Default functionality</title>
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$( function() {
$( "#dialog" ).dialog();
} );
</script>
</head>
<body>
<div id="dialog" title="Basic dialog">
<p>This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.</p>
</div>
</body>
</html>
Answer from PCasagrande on Stack Overflowmessages - Javascript Alert Box - User Experience Stack Exchange
How can I make a custom javascript alert() box like the one in the snap website? - Advanced Topics - Snap! Forum
How to edit a JavaScript alert box title? - Stack Overflow
javascript - HTML - Alert Box when loading page - Stack Overflow
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The alert box is a system object, and not subject to CSS. To do this style of thing you would need to create an HTML element and mimic the alert() functionality. The jQuery UI Dialogue does a lot of the work for you, working basically as I have described: Link.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>jQuery UI Dialog - Default functionality</title>
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$( function() {
$( "#dialog" ).dialog();
} );
</script>
</head>
<body>
<div id="dialog" title="Basic dialog">
<p>This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.</p>
</div>
</body>
</html>
I use SweetAlert, It's Awesome, You will get lots of customization option as well as all callbacks

swal("Here's a message!", "It's pretty, isn't it?");

Yes you need javascript. The simplest way is to just put this at the bottom of your HTML page:
<script type="text/javascript">
alert("Hello world");
</script>
There are more preferred methods, like using jQuery's ready function, but this method will work.
You can use a variety of methods, one uses Javascript window.onload function in a simple function call from a script or from the body as in the solutions above, you can also use jQuery to do this but its just a modification of Javascript...Just add Jquery to your header by pasting
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
to your head section and open another script tag where you display the alert when the DOM is ready i.e. `
<script>
$("document").ready( function () {
alert("Hello, world");
});
</script>
`
This uses Jquery to run the function but since jQuery is a Javascript framework it contains Javascript code hence the Javascript alert function..hope this helps...

