Why is there space between the document and my navigator?

Question

The padding and margin of the body and my navigator has been set to 0, yet there is still space between the top part of my navigator. How do I fix this?

body {
  margin: 0;
  padding: 0;
}

#header_container {
  margin: 0;
  padding: 0;
  background-color: rgb(0, 0, 119);
}

<div id="header_container">
  <ul>
    <li id="homepage_name">name</li>
    <div id="buttons">
      <li id="home">Home</a>
      </li>
      <li id="about_us">About us</a>
      </li>
      <li id="contacts">Contacts</li>
    </div>
  </ul>
</div>

Answer 1

Also need to remove margin from ul.

body {
margin: 0;
padding: 0;
}

#header_container {
margin: 0;
padding: 0;
background-color: rgb(0, 0, 119);
}
ul{
  margin:0;
}

<div id="header_container">
    <ul>
        <li id="homepage_name">name</li>
        <div id="buttons">
            <li id="home">Home</a></li>
            <li id="about_us">About us</a></li>
            <li id="contacts">Contacts</li>
        </div>
    </ul>
</div>