If you have for example a <H1> tag inside a <Div> tag,
the webpage renders a empty space between the 2 div items ("header" and "menu" below)

<div class="header">
            <h1>Website</h1>
        </div>
        <div class="menu">
            this is the next line
        </div>

 To resolve this issue try to add the following to your CSS.

h1  
{
    margin:0px;
    padding: 0px;
    }