popup.html

<!doctype html>
<!--
 This page is shown when the extension button is clicked, because the
 "browser_action" field in manifest.json contains the "default_popup" key with
 value "popup.html".
 -->
<html>
  <head>
    <title>In-Page Timer</title>
    <style>
      body {
        font-family: "Segoe UI", "Lucida Grande", Tahoma, sans-serif;
        font-size: 100%;
      }
    </style>

  </head>
  <body>
    <label for="interval_mins">Minutes: </label><input type="number" id="interval_mins" value="0"/>
    <label for="interval_secs">Seconds: </label><input type="number" id="interval_secs" value="0"/>
    <button id="btn">Start</button><br>
    <button id="pausebtn">Pause</button>
    <button id="resumebtn">Resume</button>
    <button id="resetbtn">Reset</button>
    
    <script src="popup.js"></script>
  </body>
</html>