Golang template function returns content of index.html instead of page

I try to create web application via this tutorial, but localhost:3000 shows content of index.html instead of page. What is wrong with my code?
index.html result
main.go

package main

import (
    "html/template"
    "net/http"
    "os"
)

var tpl = template.Must(template.ParseFiles("index.html"))

func indexHandler(w http.ResponseWriter, r *http.Request) {
    tpl.Execute(w, nil)
}

func main() {
    port := os.Getenv("PORT")
    if port == "" {
        port = "3000"
    }

    mux := http.NewServeMux()

    mux.HandleFunc("/", indexHandler)
    http.ListenAndServe(":"+port, mux)
}

index.html

<DOCTYPE>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Orders</title>
    <!-- <link rel="stylesheet" href="/assets/style.css"> -->
</head>
<body>
    <h1>Orders12</h1>
</body>
</html>

go version go1.22.4 windows/amd64

if I replase tmpl.Execute(w, nil) with w.Write([]byte("<h1>HELLO W</h1>")) - it wokrs, so server is ok. Maybe proble is in tmpl.Execute(w, nil)

Solution

As per the comments, the first line should be <!DOCTYPE html> (playground). See MDN for more info.