JSFiddle - React, Tailwind, and code Playground

HTML

<div id='nav'>
  <label for='speed'>&nbsp;Speed
    <br />
    <input type='number' class='C1' id='speed' placeholder='1-10' max='10' min='1' />
  </label>
  <br />
  <label for='Iron'>&nbsp;Iron
    <br />
    <input type='number' class='C1' id='Iron' placeholder='1-5' max='5' min='1' />
  </label>
  <br />
  <label for='Ice'>&nbsp;Ice
    <br />
    <input type='number' class='C1' id='Ice' placeholder='1-5' max='5' min='1' />
  </label>
  <br />
  <label for='Mass'>&nbsp;Mass
    <br />
    <input type='number' class='C1' id='Mass' placeholder='1-5' max='5' min='1' />
  </label>
  <br />
  <label for='Diameter'>&nbsp;Diameter
    <br />
    <input type='number' class='C1' id='Diameter' placeholder='1-5' max='5' min='1' />
  </label>
  <br />
  <center>
    <p id='spacer'>&nbsp;</p>
    <p id='enterSign' onclick='saveitem()'>Press Enter</p>
  </center>
  <center>
    <button id='btnReset' onclick='resetPage()'>Reset</button>
  </center>
</div>

JavaScript

var nav = document.getElementById('nav');
nav.addEventListener("keyup", function(e) {
    var el = e.target;
    if (el.className === 'C1') {
        // The element that trigger this event is an input so do check
        if (el.value) {
            el.style.borderColor = "green";
        } else {
            el.style.borderColor = "red";
        }   
    }
});