Thanks! Works.
Thanks to this addon I wrote script for Greasemonkey (https://cup.aihedy.com/ru/firefox/addon/greasemonkey/ ) that automatically adds to certain site:
(function() {
let link = location.href;
if (link.indexOf("website_that_needs_PRE_tag.com") != -1){
document.body.innerHTML = '' + document.body.innerHTML;
}
})();