LibHTML: Make <header> and <footer> be "display: block" by default

This makes http://www.motherfuckingwebsite.com/ readable :^)

Libraries/LibHTML/CSS/Default.css

@@ -49,7 +49,8 @@ frame, frameset,
 h1, h2, h3, h4,
 h5, h6, noframes,
 ol, p, ul, center,
-dir, hr, menu, pre {
+dir, hr, menu, pre,
+header, footer {
     display: block;
 }