现场演示：我的 HTML 文档

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>JavaScript Detect Internet Connection Status</title>
    <style>
        body {
            text-align: center;
            font-family: "Segoe UI", Arial, sans-serif;
        }
        #status {
            border: 2px solid;
            padding: 15px 20px 15px 40px;
            width: 160px;
            margin: 0 auto;
            border-radius: 20px;
            font-size: 30px;
            font-weight: bold;
            text-transform: uppercase;
            position: relative;
        }
        #status.online {
            color: green;
        }        
        #status.offline {
            color: red;
        }
        #status.online::before, #status.offline::before {
            width: 25px;
            height: 25px;
            content: "";
            border-radius: 15px;
            box-shadow: 0 0 8px;
            position: absolute;
            left: 20px;
            top: 25px;
        }
        #status.online::before {
            background: green;
        }
        #status.offline::before {
            background: red;
        }
    </style>
</head>
<body>
    <script>    
    var hint = document.getElementById("hint");
    
    // Defining function to update connection status
    function updateConnectionStatus() {  
        var status = document.getElementById("status");
        if(navigator.onLine) {
            status.innerHTML = "Online";
            status.classList.add("online");
            status.classList.remove("offline");                        
        } else {
            status.innerHTML = "Offline";
            status.classList.add("offline");
            status.classList.remove("online");            
        }
    }

// Attaching event handler for the load event
    window.addEventListener("load", updateConnectionStatus);
    
    // Attaching event handler for the online event
    window.addEventListener("online", function(e) {
        updateConnectionStatus();
        hint.innerHTML = "And we're back!";
    });
    
    // Attaching event handler for the offline event
    window.addEventListener("offline", function(e) {        
        updateConnectionStatus();
        hint.innerHTML = "Hey, it looks like you're offline.";
    });
    </script>
    <div id="status"></div>    
    <p>Toggle your internet connection on/off to see how it works.</p>
    <p id="hint"></p>
</body>
</html>