There is nothing that you can do with CSS.

<title>app · Streamlit</title>

This is how you could set an empty text:

document.querySelector('title').innerHTML = '';

Or you could also remove that tag:

document.querySelector('title').remove();

<link rel="shortcut icon" href="./favicon.png">

Removing the value ./favicon.png will not help, so you will need to set an empty icon:

document.querySelector('link[rel~="shortcut"]').setAttribute('href', 'data:image/x-icon');

CSS cannot touch the browser chrome. It can't change the favicon or title in the address bar / browser tab / window title bar.

It can only affect the document in the viewport.