Untitled

<!DOCTYPE html>
<html>
  <head>
    <title>Lista</title>
  </head>
  <script>
    window.onload = () =>
      document
        .getElementsByTagName("form")[0]
        .addEventListener("submit", event => submitHandler(event));

    function submitHandler(event) {
      event.preventDefault();
      addBullet();
      document.getElementById("user-todo").value = "";
    }

    function clearList() {
      document.getElementById("items").innerHTML = "";
    }

    function addBullet() {
      let value = document.getElementById("user-todo").value;
      let bullet = document.createElement("li");
      bullet.appendChild(document.createTextNode(value));

      document.getElementById("items").appendChild(bullet);
    }
  </script>
  <body>
    <h1 id="title">Lista</h1>
    <form>
      <input type="text" id="user-todo" placeholder="Lista" required />
    </form>
    <h2 id="todo-header">Rzeczy:</h2>
    <ul id="items"></ul>
    <button id="clear" onclick="clearList()">Reset</button>
    <script></script>
  </body>
</html>