How wide is the default `<body>` margin?

Solution 1:

In most major browsers, the default margin is 8px on all sides. It is defined in pixels by the user-agent-stylesheet your browser provides.

Some browsers allow you to create and use your own user-agent-stylesheet, but if you are developing a website, I would recommend staying away from changing this, since your users most likely will not have a modified stylesheet and would then see a different page than you do.

If you want to change it, you can just do this:

body {
  margin: 0px;
  padding: 0px;
  ...
}

But if you have a large project and want to be more complete, use normalize.css. It resets a lot of default values to be consistent across browsers.

Solution 2:

According to W3School's CSS reference, the default properties and values for a body tag are,

body{ display : block; margin : 8px; }

And one can quickly get the computed details of any element by accessing the Computed Pane in the Chrome Dev tools.