Site Isolation is a feature of the Google Chrome browser that adds an additional security boundary between websites by ensuring that different sites are always put into separate processes, isolated from each other. Google has now by default enabled this security feature for 99% of Chrome desktop users on Windows, Mac, Linux, and Chrome OS. The feature also offers more protection against a certain type of web browser security bug, called universal cross-site scripting (UXSS) Google is keeping a 1 percent holdback, for now, to monitor and improve performance.
Source: https://thehackernews.com/2018/07/google-chrome-site-isolation.html