Styles

The markup page references an external stylesheet document.

<!DOCTYPE html>
<html>
  <head>
    <title>Our Solar System</title>
    <!-- the link element is used to reference a stylesheet -->
    <link rel="stylesheet" href="styles/solarsystem.css" />
    ...cut...

The CSS file contains style rules for the document (solarsystem.css)

Contents of the CSS file: 
body {
  margin-left: 5%;
  margin-top: 2em;
  margin-right: 5%;
  background-image: linear-gradient(white, #ffffdd);
  background-color: #ffffdd;
  font-family: Calibri,Arial,sans-serif;
}
h1 {
  font-family: Calibri,Arial, sans-serif;
  color: #333333;
  border-bottom: 3px solid #333333;
}
ul.gallery {
  list-style: none;
  padding: 0;
  margin: 0;
}
ul.gallery li {
  margin-top: 1em;
  font-size: 1.25em;
  height: 250px;
  width: 220px;
  float: left;
  text-align: center;
  border: thin dotted #333333;
  margin: 10px;
  padding: 10px;
}
ul.gallery li img { border: none; }
footer { clear: both; margin-top: 2rem; padding: 1em; font-size: smaller;}

a:link,a:visited {text-decoration: none; color: blue; }
a:hover { text-decoration: underline; }
a:active { border: none; text-decoration: none; }