Ole

You can use CSS to change the background image of a browser tab. The favicon is the small icon that appears in the browser tab, but you can also use a larger image as a background. To do this, you need to create a favicon at the size you want, and then reference it in the head of your HTML document using the link tag.
<link rel="icon" type="image/png" href="path/to/image.png">
This will set the background image of the tab to the image located at path/to/image.png.
It is important to note that not all web browsers support the ability to display background images in tabs, and that this feature is experimental and not part of the standard of web technologies.
It's also worth mentioning that some browsers may have different sizes, resolutions, and support for certain image types. It is recommended to use a .ico file format and include several sizes of the image to support most of the popular web browsers.

The browser css is unique to the browser and can’t be overridden externally. There are a whole bunch of security reason why.
And the majority of personal browsing uses mobile devices where tabs aren’t even on display.