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<html><title>W3.CSS</title><meta name="viewport" content="width=device-width, initial-scale=1"><link rel="stylesheet" href="https://www.w3schools.com/w3css/5/w3.css"><body><div class="w3-container"> <p>The w3-opacity classs adds opacity/transparency to an element (opacity: 0.6), while w3-opacity-off turns off the transparency (opacity: 1).</p> <button onclick="myFunction()" class="w3-button w3-black">Toggle opacity (turn on and off)</button> <p id="Demo" class="w3-opacity w3-red">Demo paragraph.</p></div><script>function myFunction() { var x = document.getElementById("Demo"); if (x.className.indexOf("w3-opacity-off") == -1) { x.className += " w3-opacity-off"; } else { x.className = x.className.replace(" w3-opacity-off", ""); }}</script></body></html>