diff --git a/index.html b/index.html
index 4c927d4..d0f4b22 100644
--- a/index.html
+++ b/index.html
@@ -17,16 +17,13 @@
   <script src="codemirror/lib/util/continuelist.js"></script>
   <script src="rawinflate.js"></script>
   <script src="rawdeflate.js"></script>
+  <script src="scroll-sync.js"></script>
   <link rel="stylesheet" href="base16-light.css">
   <link rel="stylesheet" href="codemirror/lib/codemirror.css">
   <link rel="stylesheet" href="default.css">
   <style>
     body {margin: 0;}
 
-    .CodeMirror pre{
-      line-height: 16px;
-    }
-
     #in{
       position: fixed;
       top: 0;
@@ -306,9 +303,21 @@
         return '<img src="http://latex.codecogs.com/png.latex?' + encodeURIComponent(b) + '" />';
       });
 
+      var env = {};
+      var tokens = md.parse(val, env);
+
+      tokens.forEach(function(t){
+        if(t.map){
+          t.attrPush([ 'source-lines', t.map[0] + ' ' + t.map[1] ]);
+        }
+      });
+
+      var html = md.renderer.render(tokens, md.options, env);
+
+
       var out = document.getElementById('out');
       var old = out.cloneNode(true);
-      out.innerHTML = md.render(val);
+      out.innerHTML = html;
       emojify.run(out);
 
       var allold = old.getElementsByTagName("*");
@@ -361,6 +370,8 @@
     }
 
     function saveAsHtml() {
+      // TODO out.innerHTML includes source-lines and various emoji refs
+      // probably best to do a fresh md.render(editor.getValue())
       save(document.getElementById('out').innerHTML, "untitled.html");
     }
 
@@ -464,6 +475,9 @@
       update(editor);
       editor.focus();
     }
+
+
+    scrollSync(editor, document.getElementById('out'));
   </script>
 </body>
 </html>
diff --git a/scroll-sync.js b/scroll-sync.js
new file mode 100644
index 0000000..152bd38
--- /dev/null
+++ b/scroll-sync.js
@@ -0,0 +1,188 @@
+var OUTPUT_SCROLLING = 1;
+var INPUT_SCROLLING = 2;
+var EDITING = 3;
+
+
+function scrollSync(editor, output){
+
+  var h1 = document.createElement('div');
+  h1.style.background = 'red';
+  h1.style.position = 'absolute';
+  h1.style.left = '50%';
+  h1.style.right = 0;
+  h1.style.height = '1px';
+  document.body.appendChild(h1);
+
+
+  var h2 = document.createElement('div');
+  h2.style.background = 'red';
+  h2.style.position = 'absolute';
+  h2.style.right = '50%';
+  h2.style.left = 0;
+  h2.style.height = '1px';
+  document.body.appendChild(h2);
+
+  var _scrollMode;
+  var _scrollTimeout;
+  function setScrollMode(mode){
+    if(_scrollMode && _scrollMode !== mode){
+      return false;
+    }
+    _scrollMode = mode;
+    clearTimeout(_scrollTimeout);
+    _scrollTimeout = setTimeout(function(){
+      _scrollMode = null;
+    }, 300);
+    return true;
+  }
+
+
+  function outputScrolled(){
+    if(!setScrollMode(OUTPUT_SCROLLING)) return;
+
+    var st = output.scrollTop;
+    var h = output.offsetHeight;
+    var sh = output.scrollHeight;
+
+    var progress = st / (sh - h);
+
+    h1.style.top = (h * progress) + 'px';
+
+    var line = getOutputLineAt(h * progress);
+
+    scrollInputToLineAtProgress(line, progress);
+  }
+
+  function getOutputPositionMarkers(offsetByScroll){
+    var els = output.getElementsByTagName('*');
+    var st = output.scrollTop;
+    if(!offsetByScroll) st = 0;
+    var markers = [];
+    for(var i = 0, l = els.length; i < l; i++){
+      var e = els[i];
+      var sl = e.getAttribute('source-lines');
+      if(!sl) continue;
+      var bits = sl.split(' ');
+      var ot = e.offsetTop;
+      var h = e.offsetHeight;
+      var start = +bits[0];
+      var end = +bits[1];
+
+      markers.push({ y: ot - st, l: start });
+      markers.push({ y: ot + h - st, l: end - 0.5 });
+    }
+
+    markers.sort(function(a, b){
+      return a.y - b.y;
+    });
+
+    // last line end un-correction
+    markers[markers.length - 1].l += 0.5;
+
+    return markers;
+  }
+
+  function getOutputLineAt(px){
+    var markers = getOutputPositionMarkers(true);
+
+    if(markers[0].y > px){
+      return 0;
+    }
+    if(markers[markers.length-1].y < px){
+      return markers[markers.length-1].l + 0.5;
+    }
+
+    for(var i = 0; i < markers.length - 1; i += 1){
+      var a = markers[i], b = markers[i + 1];
+      if(a.y <= px && b.y > px){
+        var perc = (px - a.y) / (b.y - a.y);
+        return a.l + perc * (b.l - a.l);
+      }
+    }
+  }
+
+  function scrollInputToLineAtProgress(line, progress){
+    var ei = editor.getScrollInfo();
+    if(ei.height <= ei.clientHeight){
+      // no scrolling at all
+      return;
+    }
+
+    var padding = 30;
+
+    var fl = Math.floor(line);
+
+    var y1 = editor.heightAtLine(fl);
+    var y2 = editor.heightAtLine(fl + 1);
+    var y = y1 + (line - fl) * (y2 - y1);
+
+    // TODO clientHeight seems to be off by 30px, not sure why
+    var px = padding + progress * (ei.clientHeight - /* 2* */ padding);
+    h2.style.top = px + 'px';
+
+    editor.scrollTo(null, y - px + ei.top);
+  }
+
+  function inputScrolled(){
+    if(!setScrollMode(INPUT_SCROLLING)) return;
+
+    var ei = editor.getScrollInfo();
+    if(ei.height <= ei.clientHeight){
+      // no scrolling at all
+      return;
+    }
+
+    var padding = 30;
+
+    var st = ei.top;
+    var h = ei.clientHeight;
+    var sh = ei.height;
+
+    var progress = st / (sh - h);
+
+    var px = padding + progress * (ei.clientHeight - /* 2* */ padding);
+    h2.style.top = px + 'px';
+
+
+    var l1 = editor.lineAtHeight(px);
+
+    var y1 = editor.heightAtLine(l1);
+    var y2 = editor.heightAtLine(l1 + 1);
+
+    var lineProgress = (px - y1) / (y2 - y1);
+    if(y1 === y2){ lineProgress = 0; }
+
+    var line = l1 + lineProgress;
+    if(line < 0){ line = 0; }
+
+    scrollOutputToLineAtProgress(line, progress);
+  }
+
+  function scrollOutputToLineAtProgress(line, progress){
+    var h = output.offsetHeight;
+
+    h1.style.top = (h * progress) + 'px';
+
+    var markers = getOutputPositionMarkers();
+    for(var i = 0; i < markers.length - 1; i += 1){
+      var a = markers[i], b = markers[i + 1];
+      if(a.l <= line && b.l > line){
+        var prog = (line - a.l) / (b.l - a.l);
+        var px = a.y + prog * (b.y - a.y);
+
+        var ypx = px - (h * progress);
+        output.scrollTop = ypx;
+
+        return;
+      }
+    }
+
+    // we got to the end. Oh well.
+    output.scrollTop = output.scrollHeight;
+  }
+
+
+  output.addEventListener('scroll', outputScrolled);
+  editor.on('scroll', inputScrolled);
+
+}