How to make sure that there is a correct encoding (Python, Flask)?

Question

Python

Hello.
I write the code for returning the main page:

@app.route("/")
def get_index():
    with open("index.html", "r") as file:
        return file.read()

But for some reason, something strange is shown in the browser instead of normal Russian characters.
For example, instead Клиентof showing Клиент.
If you just open the index.html file in the browser, then everything is fine.
Same with other files. Files encoded in UTF-8.
What to do?
PS I didn't do anything extra in the code, the code is also in UTF-8 encoding.
UPD: The html file says<meta charset="utf-8">

Answer 1

Basically, I wrote this:

@app.route("/")
def get_index():
    with open("index.html", "r", encoding="utf-8") as file:
        return file.read()

And it worked. (I tracked that the file is not read correctly on the server.)
And earlier I wrote the same way, but it didn't work...