diff --git a/lib/carto/functions.js b/lib/carto/functions.js
index 021f90501..613f6c614 100644
--- a/lib/carto/functions.js
+++ b/lib/carto/functions.js
@@ -10,6 +10,24 @@ tree.functions = {
         if (rgb.some(isNaN) || isNaN(a)) return null;
         return new tree.Color(rgb, a);
     },
+    // Only require val
+    stop: function (val) {
+        if (arguments.length > 1) var color = arguments[1];
+        if (arguments.length > 2) var mode = arguments[2];
+
+        return {
+            is: 'tag',
+            val: val,
+            color: color,
+            mode: mode,
+            toString: function(env) {
+                return '\n\t<stop value="' + val.ev(env) + '"' +
+                    (color ? ' color="' + color.ev(env) + '" ' : '') +
+                    (mode ? ' mode="' + mode.ev(env) + '" ' : '') +
+                    '/>';
+            }
+        };
+    },
     hsl: function (h, s, l) {
         return this.hsla(h, s, l, 1.0);
     },
diff --git a/lib/carto/tree/call.js b/lib/carto/tree/call.js
index 13c22b6fc..07423bc10 100644
--- a/lib/carto/tree/call.js
+++ b/lib/carto/tree/call.js
@@ -29,7 +29,7 @@ tree.Call.prototype = {
         }
 
         if (this.name in tree.functions) {
-            if (tree.functions[this.name].length === args.length) {
+            if (tree.functions[this.name].length <= args.length) {
                 var val = tree.functions[this.name].apply(tree.functions, args);
                 if (val === null) {
                     env.error({
diff --git a/lib/carto/tree/definition.js b/lib/carto/tree/definition.js
index f1a80acb0..c2e730563 100644
--- a/lib/carto/tree/definition.js
+++ b/lib/carto/tree/definition.js
@@ -130,6 +130,9 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
             if (x && x.serialization && x.serialization === 'content') {
                 selfclosing = false;
                 tagcontent = attributes[j].ev(env).toXML(env, true);
+            } else if (x && x.serialization && x.serialization === 'tag') {
+                selfclosing = false;
+                tagcontent = attributes[j].ev(env).toXML(env, true);
             } else {
                 xml += attributes[j].ev(env).toXML(env) + ' ';
             }
@@ -137,7 +140,7 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
         if (selfclosing) {
             xml += '/>\n';
         } else {
-            if (tagcontent.indexOf('<Format') != -1) {
+            if (tagcontent.indexOf('<') != -1) {
                 xml += '>' + tagcontent + '</' + name + '>\n';
             } else {
                 xml += '><![CDATA[' + tagcontent + ']]></' + name + '>\n';
diff --git a/lib/carto/tree/reference.js b/lib/carto/tree/reference.js
index 1dae32eed..a10fa8fbc 100644
--- a/lib/carto/tree/reference.js
+++ b/lib/carto/tree/reference.js
@@ -163,6 +163,12 @@ ref.validValue = function(env, selector, value) {
             }
         }
         return true;
+    } else if (ref.selector(selector).type == 'tags') {
+        if (!value.value) return false;
+        for (i = 0; i < value.value[0].value.length; i++) {
+            if (value.value[0].value[i].is !== 'tag') return false;
+        }
+        return true;
     } else if (ref.selector(selector).type == 'functions') {
         // For backwards compatibility, you can specify a string for `functions`-compatible
         // values, though they will not be validated.
diff --git a/package.json b/package.json
index 5b63d71a9..3ee3129f4 100644
--- a/package.json
+++ b/package.json
@@ -36,7 +36,7 @@
   },
   "dependencies": {
     "underscore": "~1.4.3",
-    "mapnik-reference": "~5.0.3",
+    "mapnik-reference": "https://github.com/mapnik/mapnik-reference/archive/raster-colorizer.tar.gz",
     "xml2js": "~0.1.13",
     "optimist": "~0.3.5"
   },
diff --git a/test/rendering/rastercolorizer.mml b/test/rendering/rastercolorizer.mml
new file mode 100644
index 000000000..719e22a59
--- /dev/null
+++ b/test/rendering/rastercolorizer.mml
@@ -0,0 +1,14 @@
+{
+    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+    "Stylesheet": [
+        "rastercolorizer.mss"
+    ],
+    "Layer": [{
+        "name": "world",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/test_data/shape_demo.zip",
+            "type": "shape"
+        }
+    }]
+}
\ No newline at end of file
diff --git a/test/rendering/rastercolorizer.mss b/test/rendering/rastercolorizer.mss
new file mode 100644
index 000000000..c13c9403f
--- /dev/null
+++ b/test/rendering/rastercolorizer.mss
@@ -0,0 +1,10 @@
+#world {
+  raster-opacity:0;
+  raster-colorizer-default-mode:discrete;
+  raster-colorizer-default-color:#f00;
+  raster-colorizer-epsilon:0.05;
+  raster-colorizer-stops:
+    stop(5, #f00)
+    stop(10, #f40, linear)
+    stop(10, #f80);
+}
\ No newline at end of file