How can I remove space (margin) around an HTML header?

Question

There is always some space above my header. How can I remove it?

My HTML and CSS code is given below.

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

header{
  margin: 0px;
  padding: 0px;
  height: 20em;
  background-color: #C0C0C0;
}

<header>
  <h1>OQ Online Judge</h1>
  <form action="<?php echo base_url();?>/index.php/base/si" method="post">
    <label for="email1">E-mail : </label><input type="text" name="email" id="email1">
    <label for="password1">Password : </label><input type="password" name="password" id="password1">
    <input type="submit" name="submit" value="Login">
  </form>
</header>

Answer 1

Try:

h1 {
    margin-top: 0;
}

You're seeing the effects of margin collapsing.