A padding value can't be negative.

See:
http://www.w3.org/TR/CSS21/box.html#padding-properties
http://www.w3.org/wiki/CSS/Properties/padding

Closes: #159

Author: Onno van der Zee

src/css/ValidationTypes.js

@@ -177,7 +177,7 @@ var ValidationTypes = {
         },
 
         "<padding-width>": function(part){
-            return this["<length>"](part) || this["<percentage>"](part);
+            return part.value >= 0 && (this["<length>"](part) || this["<percentage>"](part));
         },
 
         "<shape>": function(part){

tests/css/CSSParserTests.htm

@@ -1797,6 +1797,50 @@ <h1>CSS Parser Tests</h1>
 
         },
 
+        testPaddingLeft1: function(){
+            var parser = new Parser({ strict: true});
+            parser.addListener("property", function(event){
+                assert.isNull(event.invalid);
+            });
+            var result = parser.parse(".foo { padding-left: 6px; }");
+
+        },
+
+        testPaddingLeft2: function(){
+            var parser = new Parser({ strict: true});
+            parser.addListener("property", function(event){
+                assert.isNull(event.invalid);
+            });
+            var result = parser.parse(".foo { padding-left: 3%; }");
+        },
+
+        testPaddingLeft3: function(){
+            var parser = new Parser({ strict: true});
+            parser.addListener("property", function(event){
+                assert.isNull(event.invalid);
+            });
+            var result = parser.parse(".foo { padding-left: inherit; }");
+
+        },
+
+        testInvalidPaddingLeft1: function(){
+            var parser = new Parser({ strict: true});
+            parser.addListener("property", function(event){
+                assert.isNotNull(event.invalid);
+                assert.areEqual("Expected length greater than 0, percentage greater than 0 or inherit but found '-10px'.", event.invalid.message);
+            });
+            var result = parser.parse(".foo { padding-left: -10px; }");
+        },
+
+        testInvalidPaddingLeft2: function(){
+            var parser = new Parser({ strict: true});
+            parser.addListener("property", function(event){
+                assert.isNotNull(event.invalid);
+                assert.areEqual("Expected length, percentage or inherit but found 'auto'.", event.invalid.message);
+            });
+            var result = parser.parse(".foo { padding-left: auto; }");
+        },
+
         testBorderWidth1: function(){
             var parser = new Parser({ strict: true});
             parser.addListener("property", function(event){

tests/css/Validation.js

@@ -1054,6 +1054,22 @@
         }
     }));
 
+    suite.add(new ValidationTestCase({
+        property: "padding-left",
+
+        valid: [
+            "6px",
+            "3%",
+            "inherit"
+        ],
+
+        invalid: {
+            "-10px" : "Expected (<padding-width> | inherit) but found '-10px'.",
+            "-3%"   : "Expected (<padding-width> | inherit) but found '-3%'.",
+            "auto"   : "Expected (<padding-width> | inherit) but found 'auto'."
+        }
+    }));
+
     suite.add(new ValidationTestCase({
         property: "will-change",