Load CSS JS from Assets folder

I am not a mobile developer, but I am a web developer that did write some webview pages for my mobile developer colleagues.

As far as I know, you are not able to access file system in webview. However, you can let your app cache the css / js files.

viewer.getSettings().setCacheMode(WebSettings.LOAD_DEFAULT)

(This is from an answer here on stackoverflow) (and here is the document on cache settings)

By using the default cache settings, the CSS / JS files will be cached after downloaded in the first time, as it was cached in normal browser. So you can simply use

<link href="https://your.domain/css/style.css" rel="stylesheet" type="text/css">

to achieve the faster page load you want.

If you want to load css and js from local asset folder then first you need to download your webpage and then after you need to pass in web browser like following way,

Download Data Like using this :

public String getHtmlContent(String urlToLoad) {
        String outputStr = "";
        BufferedReader inputString = null;
        try {
            URL urlLoad = new URL(urlToLoad);
            inputString = new BufferedReader(new InputStreamReader(urlLoad.openStream()));
            String str;
            while ((str = inputString.readLine()) != null) {
                outputStr += str;
            }
        } catch (MalformedURLException e) {
        } catch (IOException e) {
        } finally {
            if (inputString != null) {
                try {
                    inputString.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return outputStr;
    }

Then after you need to put your js and css file inside of asset folder and then need to define a url in web page like following

<script src="file:///android_asset/jquery.min.js" type="text/javascript"></script>

You need set all the url using like file:///android_asset/ then after your css or js name,

After all thing finish you need to set your webpage content with webview like following

String webData = getHtmlContent("http://webisteaddress.com/index.html");
    mWebView.loadDataWithBaseURL("file:///android_asset/", webData, "text/html", "utf-8", "");

Use this function to load CSS & JavaScript inside your WebView: inject both CSS & JavaScript inside onCreate() on WebViewClient:

     webView.setWebViewClient(new WebViewClient() {
        @Override
        public void onPageFinished(WebView view, String url)
        {
            injectJavaScript(view);
            injectCSS();

         }

Create two methods to inject JavaScript & CSS(from res/raw):

    private boolean injectJavaScript(WebView view){
        view.loadUrl("javascript:(function() { " +
            "var head = document.getElementsByTagName('header')[0];"
            + "head.parentNode.removeChild(head);" + "console.log('true');"+
            "})()");
        view.loadUrl("javascript:(function() { " +
            "var footer = document.getElementsByTagName('footer')[0];"
            + "footer.parentNode.removeChild(footer);" +
            "})()");
        view.loadUrl("javascript:(function() { " +
            "var nav = document.getElementsByTagName('nav')[0];"
            + "nav.parentNode.removeChild(nav);" +
            "})()");
        view.loadUrl("javascript:(function() { " +
            "var set = document.getElementsByClassName('banner');"
            + "set[0].style.margin = '0px';" +
            "})()");
    return true;
    }
    private void injectCSS() {
    try {
        InputStream inputStream = getResources().openRawResource(R.raw.css);
        byte[] buffer = new byte[inputStream.available()];
        inputStream.read(buffer);
        inputStream.close();
        String encoded = Base64.encodeToString(buffer, Base64.NO_WRAP);
        wv1.loadUrl("javascript:(function() {" +
                "var parent = document.getElementsByTagName('head').item(0);" +
                "var style = document.createElement('style');" +
                "style.type = 'text/css';" +
                "style.innerHTML = window.atob('" + encoded + "');" +
                "parent.appendChild(style)" +
                "})()");

    } catch (Exception e) {
        e.printStackTrace();
    }
}

This worked for me like a charm.

Loading CSS using <link rel="stylesheet" href=""/> is going to be deprecated in March 2018. I just got a warning message for this in Developer Console today.

This is because nothing renders on the page till all the CSS has loaded.

So instead, the suggestion is that we load the CSS using JavaScript, and have a small inline stylesheet to render the basic look of the page; plus separate stylesheets for each section of the page, which are called from the <body> rather than the <head>.