Introduction

The goal of this tutorial is to guide you through the steps required to design web applications that are compatible across different platforms. You will learn how to create user interfaces that offer a consistent user experience regardless of the platform on which they are used.

By the end of this tutorial, you will have learned:
- Key concepts and techniques for cross-platform web design
- How to write code that works across different platforms
- Best practices for cross-platform design

Prerequisites:
- Basic understanding of HTML, CSS, and JavaScript
- Familiarity with a text editor and a web browser for testing

Step-by-Step Guide

Responsive Design: This is the technique of designing your website so that it adjusts to the size of the screen. Use CSS media queries to apply different styles for different screen sizes.

Progressive Enhancement: Build your web application for the lowest common denominator first (like a text-only browser), then progressively add enhancements for more advanced browsers or devices.

Feature Detection: Use JavaScript libraries like Modernizr to detect whether a browser supports a particular feature. If it doesn't, you can provide a fallback.

Use Frameworks and Libraries: Frameworks like Bootstrap, Foundation, and libraries like jQuery help abstract away some of the differences between browsers.

Testing: Test your web application on as many different devices and browsers as you can.

Code Examples

Example 1: CSS Media Queries

/* CSS for screens smaller than 600px */
@media only screen and (max-width: 600px) {
    body {
        background-color: lightblue;
    }
}

In this example, if the screen size is less than 600px, the background color of the body changes to light blue.

Example 2: Feature Detection with Modernizr

if (Modernizr.canvas) {
  // browser supports HTML5 canvas
  var canvas = document.getElementById('myCanvas');
  var ctx = canvas.getContext('2d');
} else {
  // canvas not supported, provide fallback
}

In this example, we use Modernizr to check if the browser supports HTML5 canvas. If it does, we get the drawing context. If not, we could provide a fallback.

Summary

In this tutorial, you learned about the importance of designing for cross-platform compatibility. We went through the concepts of Responsive Design, Progressive Enhancement, Feature Detection, and the use of Frameworks and Libraries. We also emphasized the importance of testing on different devices and browsers.

Next, you could delve deeper into each of the concepts we touched upon. You could also start practicing by designing your own cross-platform web application.

Practice Exercises

1. Design a simple web page that changes its layout based on the screen size.
2. Use Modernizr to provide a fallback for the HTML5 audio tag in browsers that do not support it.

Here are some solutions:

1. Use CSS media queries to change the layout based on the screen size.

@media only screen and (max-width: 600px) {
    /* Your CSS for small screens here */
}

1. Use Modernizr for feature detection and provide an alternative if HTML5 audio is not supported.

if (Modernizr.audio) {
  // Use HTML5 audio
} else {
  // Provide fallback
}

As further practice, try adding more features to your web page and testing it on different devices and browsers.