This seems to be showing the same behavior on Chrome,Firefox and IE. Adding proper component for someone from the respective team to have a look into this and provide more inputs.

Header: Content-Type: text/html; charset=UTF-8

Page: <meta http-equiv="Content-Type" content="text/html; charset=windows-1251">

content.characterSet ends up as: "UTF-8"

HTML spec has:

https://html.spec.whatwg.org/multipage/parsing.html#determining-the-character-encoding

Step 3: "If the transport layer specifies a character encoding, and it is supported, return that encoding with the confidence certain, and abort these steps."

Which is to say: if the headers specify an encoding, that is taken as the encoding regardless of what the page content says. So behavior is per spec, and Chrome behaves the same as other browsers.

Spec compliance is fine... except that means this website will be forever inaccessible for Chrome browser users. 

Other browsers have the workaround (manual page encoding selection) to bypass the wrong page encoding - Chrome doesn't. 

"I can't read the content of this page in any possible way" - from the user point of view the browser IS broken. 

Broken HTML is too common of feature to ignore,

Regards,
Dennis