<!DOCTYPE html>

<html>

<head>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>

<script>

$(document).ready(function () {

$("#toggleBtn").click(function () {

$("h1, h2, p, div").toggleClass("green");

});

$("#addClassedBtn").click(function () {

$("h1, h2, p").addClass("blue");

$("div").addClass("important");

});

});

</script>

<style>

.green {

color: green;

}

.important {

font-weight: bold;

font-size: xx-large;

}

.blue {

color: blue;

}

</style>

</head>

<body>

<h1>Heading 1</h1>

<h2>Heading 2</h2>

<p>This is a paragraph.</p>

<p>This is another paragraph.</p>

<div>This is some important text!</div><br>

<button id="toggleBtn">Toggle class</button>

<button id="addClassedBtn">Add classes to elements</button>

</body>

</html>