Writing to Firebase Database from HTML

I am trying to write some data to Firebase from html. This process works; however, sometimes I get a 503 error and sometimes I also get a "deprecation" notice and also, when I click "submit" sometimes there's a long lag until the submission is complete. Is something written in correct or the process incorrect?

  <script src="https://www.gstatic.com/firebasejs/6.2.4/firebase-app.js"></script>
  <script src="https://www.gstatic.com/firebasejs/6.2.4/firebase-firestore.js"></script>
  <script src="https://www.gstatic.com/firebasejs/3.1.0/firebase-database.js"></script>
  <script src="https://www.gstatic.com/firebasejs/4.1.2/firebase.js"></script>

 <script>
     var firebaseConfig = {
         apiKey: "AIzaSy333333CoLQMOkxO3rYLiywsrib6nCOWFBZv_xI0",
         authDomain: "dem33333otestingdata.firebaseapp.com",
         databaseURL: "https://dem33333otestingdata.firebaseio.com",
         storageBucket:"dem33333otestingdata.appspot.com",
     };
     firebase.initializeApp(firebaseConfig);

     function writeData() {
         firebase.database().ref("User").set({
             name:document.getElementById("nameField").value,
             age:document.getElementById("ageField").value
         });
     }

      </script>
      <h1>User Databse</h1>
      <input type="text" placeholder="name" id="nameField">
      <input type="text" placeholder="age" id="ageField">

      <button onclick = "writeData()">Submit</button>

Just try removing this script tag from your HTML file as this has been deprecated

  <script src="https://www.gstatic.com/firebasejs/4.1.2/firebase.js"></script>

and replace the following

  <script src="https://www.gstatic.com/firebasejs/3.1.0/firebase-database.js"></script>

with the latest CDN

<script src="https://www.gstatic.com/firebasejs/6.2.4/firebase-database.js"></script>