improve pagination while printing

Avoid widows and orphans, page breaks after headlines (fixes upstream iainc#33)

GitHub.iatemplate/Contents/Resources/github.css

@@ -61,11 +61,50 @@ html {
 @media print {
   .markdown-body {
     --horizontal-padding: 75px;
+    widows: 2;      /* avoid single lines at... */
+    orphans: 2;     /* ...begin and end of page */
   }
   .markdown-body pre code {
     white-space: pre-wrap;
   }
-}
+
+  /* avoid page breaks after headlines */
+  .markdown-body h1,
+  .markdown-body h2,
+  .markdown-body h3,
+  .markdown-body h4,
+  .markdown-body h5,
+  .markdown-body h6 {
+    /* this should be simple but due to WebKit bug https://bugs.webkit.org/show_bug.cgi?id=5097 it isn't: 
+         page-break-after: avoid;   /* old syntax, not working in webkit 
+         break-after: avoid-page;   /* new syntax, not implemented in webkit 
+       instead, increase the calculated height of headlines by using an invisible element and forbid breaks within: */
+    page-break-inside: avoid;    
+  }  
+  .markdown-body h1::after,
+  .markdown-body h2::after,
+  .markdown-body h3::after,
+  .markdown-body h4::after,
+  .markdown-body h5::after,
+  .markdown-body h6::after {
+    content: "";
+    display: block;
+    height: 50px;
+    margin-bottom: -50px;  
+  }
+  /* github-markdown.css does define bottom padding and border for h1 / h2 which conflicts with this hack 
+     remove these and define them top of the workaround instead */
+  .markdown-body h1,
+  .markdown-body h2 {
+    padding-bottom: unset;
+    border-bottom: unset;
+  }
+  .markdown-body h1::after,
+  .markdown-body h2::after {
+    margin-top: 0.3em;
+    border-top: 1px solid #eaecef;
+  }
+}  
 
 /* Fix for paragraph list items. GitHub embeds input into the first paragraph, iA Writer doesn’t. */
 .markdown-body .task-list-item {