How to add CSS for HTML5 Semantic Elements?

Answer

Look at your existing CSS styles:

div#header,div#footer,div#content,div#post {
    border:1px solid grey;margin:5px;margin-bottom:15px;padding:8px;background-color:white;
}
div#header,div#footer {
    color:white;background-color:#444;margin-bottom:5px;
}
div#content {
    background-color:#ddd;
}
div#menu ul {
    margin:0;padding:0;
}
div#menu ul li {
    display:inline; margin:5px;
}

Duplicate with equal CSS styles for HTML5 semantic elements:

header,footer,section,article {
    border:1px solid grey;margin:5px;margin-bottom:15px;padding:8px;background-color:white;
}
header,footer {
    color:white;background-color:#444;margin-bottom:5px;
}
section {
    background-color:#ddd;
}
nav ul  {
    margin:0;padding:0;
}
nav ul li {
    display:inline; margin:5px;
}