Default margin of a body

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
  <head>
    <meta http-equiv=Content-Type
             content='text/html;charset=utf-8'>
    <title>my title</title>

   <style type="text/css">

     body {
            border: 30px solid green;
          }

    </style>
  </head>

  <body>

    <p>my paragraph</p>

  </body>
</html>

The code above has no body margin.
So it displays with default body margin.

What is the default margin of a body?
Is it 10px, 15px, or some other px?

The default depends on the browser and some browsers use default padding on <body> instead of a default margin.

Depends on the browser. Some have just padding, some just margin for the body. What a lot of people do is put this at the beginning of their stylesheets:

* {
  margin:0;
  padding:0;
}

The body is not the only element with default margins/padding. That makes them 0 for all the elements so you can then specify your own with a bit less hassle.

Thank you. The unvesal default(*), I think, is cool.