Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/cssstyle/lib/properties/fontSize.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var TYPES = require('../parsers').TYPES;
+var valueType = require('../parsers').valueType;
+
+var absoluteSizes = ['xx-small', 'x-small', 'small', 'medium', 'large', 'x-large', 'xx-large'];
+var relativeSizes = ['larger', 'smaller'];
+
+module.exports.isValid = function(v) {
+  var type = valueType(v.toLowerCase());
+  return (
+    type === TYPES.LENGTH ||
+    type === TYPES.PERCENT ||
+    (type === TYPES.KEYWORD && absoluteSizes.indexOf(v.toLowerCase()) !== -1) ||
+    (type === TYPES.KEYWORD && relativeSizes.indexOf(v.toLowerCase()) !== -1)
+  );
+};
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('font-size', v);
+  },
+  get: function() {
+    return this.getPropertyValue('font-size');
+  },
+  enumerable: true,
+  configurable: true,
+};