Copy and add to bookmarks or open in the URL bar:
data:text/html,<body><style>textarea{width:99%;height:49%;}body{margin:0;}textarea{width:99%;font-size:18px;padding:0.5em}</style><textarea placeholder="Decoded" id="d"></textarea><textarea placeholder="Encoded" id="e"></textarea><script>document.querySelectorAll("textarea").forEach(t=>t.addEventListener("keyup",function(e){switch(e.target.id){case"d":document.getElementById("e").value=encodeURIComponent(e.target.value);break;case"e":document.getElementById("d").value=decodeURIComponent(e.target.value);break;}}))</script></body>
Inspired by awfice