How to detect the visitor's IP address using HTML?

You can't do it through HTML. However, you can find the IP address of a visitor through PHP.

$ip=$_SERVER['REMOTE_ADDR'];

The following script may be useful to you. You can copy it and save it as {whateveryouwant}.php

<?php

echo "Your IP is";

echo $_SERVER["REMOTE_ADDR"];

function get_ip_address() {
  // check for shared internet/ISP IP
  if (!empty($_SERVER['HTTP_CLIENT_IP']) && $this->validate_ip($_SERVER['HTTP_CLIENT_IP']))
   return $_SERVER['HTTP_CLIENT_IP'];

  // check for IPs passing through proxies
  if (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
   // check if multiple ips exist in var
    $iplist = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);
    foreach ($iplist as $ip) {
     if ($this->validate_ip($ip))
      return $ip;
    }
   }

  if (!empty($_SERVER['HTTP_X_FORWARDED']) && $this->validate_ip($_SERVER['HTTP_X_FORWARDED']))
   return $_SERVER['HTTP_X_FORWARDED'];
  if (!empty($_SERVER['HTTP_X_CLUSTER_CLIENT_IP']) && $this->validate_ip($_SERVER['HTTP_X_CLUSTER_CLIENT_IP']))
   return $_SERVER['HTTP_X_CLUSTER_CLIENT_IP'];
  if (!empty($_SERVER['HTTP_FORWARDED_FOR']) && $this->validate_ip($_SERVER['HTTP_FORWARDED_FOR']))
   return $_SERVER['HTTP_FORWARDED_FOR'];
  if (!empty($_SERVER['HTTP_FORWARDED']) && $this->validate_ip($_SERVER['HTTP_FORWARDED']))
   return $_SERVER['HTTP_FORWARDED'];

  // return unreliable ip since all else failed
   return $_SERVER['REMOTE_ADDR'];
 }

function validate_ip($ip) {
     if (filter_var($ip, FILTER_VALIDATE_IP, 
                         FILTER_FLAG_IPV4 | 
                         FILTER_FLAG_IPV6 |
                         FILTER_FLAG_NO_PRIV_RANGE | 
                         FILTER_FLAG_NO_RES_RANGE) === false)
         return false;
     self::$ip = $ip;
     return true;
 }
?>

You can't.

HTML is a markup language, not a programming language. it doesn't "do" anything - it just structures content.

You need to use a programming language, such as PHP, ASP, etc.