Default size of header tags

Currently h1 is smaller than the normal text on the page, which is
more certainly incorrect.  There are suggested default values in
appendix D of the css 2 spec (http://www.w3.org/TR/CSS21/sample.html),
which seem to work much better for h1-h6:

           }

 @media multifont {
-      h1  { font-size : +12pt }
-      h2  { font-size : +6pt  }
-      h3  { font-size : +4pt  }
-      h5  { font-size : -2pt  }
-      h6  { font-size : -4pt  }
+      h1  { font-size : 2em; margin: .67em 0 }
+      h2  { font-size : 1.5em; margin: .75em 0 }
+      h3  { font-size : 1.17em; margin: .83em 0 }
+      h5  { font-size : .83em; margin: 1.5em 0 }
+      h6  { font-size : .67em; margin: 1.67em 0 }
 }

 /* This causes problems with Emacs 19 */