swift convert html to pdf but the CSS formatting disappear

I am now have UIWebView in my view controller , and I have the html file ( Template.html, including the css inside),using the Mustache to load the data to the html, just like this:

let template = Template(named: "Template")!
let rendering = template.render(Box(self.data))!

and then show it on the webview using this

self.webView.loadHTMLString(rendering, baseURL: nil))
self.webView.scalesPageToFit = true;

all it works well. Then I wanna transform this html to pdf and also show on the same web view, but it looks that all my css layout doesn't work anymore. It only show me the html text and without layout setting by the css. I used this method to convert to pdf: http://ift.tt/1yVWGFb

and the only change is

let fmt = UIMarkupTextPrintFormatter(markupText: rendering)
render.addPrintFormatter(fmt, startingAtPageAtIndex: 0)

and then save the pdf, and load it again by the web view

let documentsPath = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as! NSString
var pdfFile = documentsPath.stringByAppendingPathComponent("invoice.PDF")

pdfData.writeToFile(pdfFile, atomically: true)
let url : NSURL! = NSURL(string: pdfFile)
webView.loadRequest(NSURLRequest(URL: url))

But now the pdf has no layout provided by the css(such like the color, the position), but still has the html layout(such like the <\br>), I don't know why, please help me!!! Thanks !!!