function getCssPropertyValue(cssStyle, propName) {
  return genericMethodCall(
             Methods.GET_PROPERTY_VALUE, cssStyle,
             cssStyle.getPropertyValue ? 'getPropertyValue' : 'getAttribute',
             [propName]) ||
      '';
}
Provides a way cross-browser way to get a CSS value from a CSS declaration.
@param {!CSSStyleDeclaration} cssStyle A CSS style object.
@param {string} propName A property name.
@return {string} Value of the property as parsed by the browser.
@supported IE8 and newer.