[css] How can I remove space (margin) above HTML header?

I am creating a website.

I have written the HTML part and now I am writing the stylesheet. But there is always some space above my header. How can I remove it?

My HTML and CSS code is given below.

body{_x000D_
  margin: 0px;_x000D_
  padding: 0px;_x000D_
}_x000D_
_x000D_
header{_x000D_
  margin: 0px;_x000D_
  padding: 0px;_x000D_
  height: 20em;_x000D_
  background-color: #C0C0C0;_x000D_
}

<header>_x000D_
  <h1>OQ Online Judge</h1>_x000D_
  <form action="<?php echo base_url();?>/index.php/base/si" method="post">_x000D_
    <label for="email1">E-mail : </label><input type="text" name="email" id="email1">_x000D_
    <label for="password1">Password : </label><input type="password" name="password" id="password1">_x000D_
    <input type="submit" name="submit" value="Login">_x000D_
  </form>_x000D_
</header>

To prevent unexpected margins and other browser-specific behavior in the future, I'd recommend to include reset.css in your stylesheets.

Be aware that you'll have to set the h[1..6] font size and weight to make headings look like headings again after that, and many other things.

I solved the space issue by adding a border and removing is by setting a negative margin. Do not know what the underlying problem is though.

header {
  border-top: 1px solid gold !important;
  margin-top: -1px !important;
}

This css allowed chrome and firefox to render all other elements on my page normally and remove the margin above my h1 tag. Also, as a page is resized em can work better than px.

h1 {
  margin-top: -.3em;
  margin-bottom: 0em;
}

It is probably the h1 tag causing the problem. Applying margin: 0; should fix the problem.

But you should use a CSS reset for every new project to eliminate browser consistencies and problems like yours. Probably the most famous one is Eric Meyer's: http://meyerweb.com/eric/tools/css/reset/

It is good practice when you start creating website to reset all the margins and paddings. So I recommend on start just to simple do:

* { margin: 0, padding: 0 }

This will make margins and paddings of all elements to be 0, and then you can style them as you wish, because each browser has a different default margin and padding of the elements.

Just for completeness, changing overflow to auto/hidden should do the trick too.

body {_x000D_
  margin: 0px;_x000D_
  padding: 0px;_x000D_
}_x000D_
_x000D_
header {_x000D_
  margin: 0px;_x000D_
  padding: 0px;_x000D_
  height: 20em;_x000D_
  background-color: #C0C0C0;_x000D_
  overflow: auto;_x000D_
}

<header>_x000D_
  <h1>OQ Online Judge</h1>_x000D_
_x000D_
  <form action="<?php echo base_url();?>/index.php/base/si" method="post">_x000D_
    <label for="email1">E-mail :</label>_x000D_
    <input type="text" name="email" id="email1">_x000D_
    <label for="password1">Password :</label>_x000D_
    <input type="password" name="password" id="password1">_x000D_
    <input type="submit" name="submit" value="Login">_x000D_
  </form>_x000D_
</header>

Try margin-top:

<header style="margin-top: -20px;">
    ...

Edit:

Now I found relative position probably a better choice:

<header style="position: relative; top: -20px;">
    ...