How to use HTML and CSS as a Java application GUI?

It's not really feasible. Rich clients in Java are done using Swing or SWT.

If you want to use HTML/CSS for your user interface, you need to use the server/client model. It can be as simple as creating a local server and launching a browser that connects to it, but it would still be that model.

If you absolutely need to have HTML/CSS as your UI framework and can't go to a server/client model, your best bet is probably looking at something like Google Native Client, but that uses C/C++ bindings on the backend. I haven't used Native Client so I can't personally give much more information on that front.

Edit to add:

One option is to embed a native browser into your Swing app using something like: http://djproject.sourceforge.net/ns/

There are some pure Java HTML renderers, however, they most likely won't be fully HTML5/CSS3 compliant, let alone possibly have Javascript bugs as well.

See here for some of those options: Pure Java HTML viewer/renderer for use in a Scrollable pane

Like @Reverand Gonzo says, you will need some form of server/client. But you could easily embed a Jetty server into a Java app and then use GWT for your client code.

You can embed web browser component into your Java Swing/JavaFX Desktop application that displays GUI built with HTML5+CSS+JavaScript. You can see an article that describes how to do this at https://jxbrowser-support.teamdev.com/docs/tutorials/cross-desktop-apps.html

One of the Java Swing/JavaFX libraries that allows embedding Chromium into Java applications is JxBrowser. Using JxBrowser API you can load any web page and work with its DOM and JavaScript. You can even call Java methods from JavaScript code and vice versa. For example:

import com.teamdev.jxbrowser.chromium.Browser;
import com.teamdev.jxbrowser.chromium.JSFunctionCallback;
import com.teamdev.jxbrowser.chromium.JSObject;
import com.teamdev.jxbrowser.chromium.JSValue;
import com.teamdev.jxbrowser.chromium.events.FinishLoadingEvent;
import com.teamdev.jxbrowser.chromium.events.LoadAdapter;

public class JavaScriptJavaSample {
    public static void main(String[] args) {
        Browser browser = new Browser();
        browser.addLoadListener(new LoadAdapter() {
            @Override
            public void onFinishLoadingFrame(FinishLoadingEvent event) {
                if (event.isMainFrame()) {
                    Browser browser = event.getBrowser();
                    JSObject window = (JSObject)
                            browser.executeJavaScriptAndReturnValue("window");
                    window.setProperty("MyFunction", new JSFunctionCallback() {
                        @Override
                        public Object invoke(Object... args) {
                            for (Object arg : args) {
                                System.out.println("arg = " + arg);
                            }
                            return "Hello!";
                        }
                    });
                    JSValue returnValue = browser.executeJavaScriptAndReturnValue(
                            "MyFunction('Hello JxBrowser!', 1, 2, 3, true);");
                    System.out.println("return value = " + returnValue);
                }
            }
        });
        browser.loadURL("about:blank");
    }
}

You can bring in the power of HTML,CSS,JavaScript into your Swing app using JFXPanel to embed JavaFX WebView. Have a look at the SimpleSwingBrowser demo in this link:https://docs.oracle.com/javase/8/javafx/interoperability-tutorial/swing-fx-interoperability.htm

WebView allows to call JavaScript functions from Java and vice versa. It is also a nice way to enhance your legacy Java app with web technologies.