luau/docs/_includes/repl.html

<form>
    <div>
        <label>Script:</label>
        <br>
        <textarea rows="10" cols="70" id="script">print("Hello World!")</textarea>
        <br><br>
        <button onclick="clearInput(); return false;">
            Clear Input
        </button>
        <button onclick="executeScript(); return false;">
            Run
        </button>
    </div>
    <br><br>
    <div>
        <label>Output:</label>
        <br>
        <textarea readonly rows="10" cols="70" id="output"></textarea>
        <br><br>
        <button onclick="clearOutput(); return false;">
            Clear Output
        </button>
    </div>
</form>

<script>
    function output(text) {
        document.getElementById("output").value += "[" + new Date().toLocaleTimeString() + "] " + text.replace('stdin:', '') + "\n";
    }

    var Module = {
        'print': function (msg) { output(msg) }
    };

    function clearInput() {
        document.getElementById("script").value = "";
    }

    function clearOutput() {
        document.getElementById("output").value = "";
    }

    function executeScript() {
        var err = Module.ccall('executeScript', 'string', ['string'], [document.getElementById("script").value]);
        if (err) {
            output('Error:' + err.replace('stdin:', ''));
        }
    }
</script>
<script async src="https://github.com/Roblox/luau/releases/download/0.505/Luau.Web.js"></script>