It'd be really handy to be able to have individual tabs logged in as different users on the same website. Right now, you can use Private Browsing, but the downside of that is that the state is per tab and not persistent, whereas the Firefox plugin I mentioned lets you configure, effectively, different namespaces for cookies and so on that are all persistent. This is *really* handy for web development, and sometimes also handy for other reasons (e.g. if you have two accounts for a system that uses web-based authentication, then being able to keep two separate sets of state is very handy indeed, otherwise you may find yourself constantly logging in and out). This is the Firefox extension I'm referring to: https://addons.mozilla.org/en-GB/firefox/addon/multi-account-containers/
Thank you for the report! This would be a browser feature, WebKit has all functionality necessary to implement this. Please file this suggestion to the maker of your browser (for Safari, this would be https://feedbackassistant.apple.com). I know that some people use Safari and Safari Technology Preview for two different contexts. This is not a perfect solution, but gets one closer to the ideal setup.