https://imgur.com/a/zkoIzLh
<!DOCTYPE html>
<html>
<meta charset="UTF-8">
</html>
<script>
fetch('/data.php').then(r => r.text()).then(r => console.log(r))
</script>
<?php
header('Content-Type: text/html; charset=windows-1251');
echo 'Привет русские буквы';
https://imgur.com/a/GxkNIHF