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

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");
    }
}