My header does not fully covers my page making the background color overlaps them both together.

```
<!DOCTYPE html>
<html>
<head>
<style>
#header {
    background-color: black;
    background-size: 100% auto;
    margin-left: 0;
    margin-top: 0;
}
</style>
</head>

<body style="background-color: blue;">

        <div id="header">
    <p>hi</p>
    </div>

</body>
</html>

```