Extends the use of the "opener" rel type to same-window navigations to signal to the browser that the destination page will open a new window and the referring page expects to be able to access it. Some user agents perform browsing context group changes on navigation that aren't strictly necessary for security in order to improve performance. This results in named window reuse not being possible across a back navigation. Annotating an anchor element with "opener" will signal to the user agent that performing a browsing context group change would break a future opener relationship.
Some browsers perform a browsing context group (BCG) swap on navigations for performance reasons. In certain scenarios (see explainer), this can cause web-facing breakage of named window reuse. A site may intend to create one popup window and then reuse it for subsequent navigations, but the browser will instead create a new tab each time. Currently, intentionally making a site bfcache-ineligible acts as a workaround. It's desirable to have an opt-out mechanism that allows authors of affected web pages to prevent breakage for their users, without excessive performance costs.
Explainers: https://github.com/explainers-by-googlers/future-browsing-context-group-dependency-hint