The performance benefits of rel=noopener -

If you have links to another origin, you should use rel="noopener", especially if they open in a new tab/window.

Without this, the new page can access your window object via window.opener. Thankfully the origin security model of the web prevents it reading your page, but no-thankfully some legacy APIs mean it can navigate your page to a different URL using window.opener.location = newURL.

Most browsers are multi-process with the exception of Firefox (and they're working on it). Each process has multiple threads, including what we often call the "main" thread. This is where parsing, style calculation, layout, painting and non-worker JavaScript runs. This means JavaScript running on one domain runs on a different thread to a window/tab running another domain.

However, due to the synchronous cross-window access the DOM gives us via window.opener, windows launched via target="_blank" end up in the same process & thread. The same is true for iframes and windows opened via

rel="noopener" prevents window.opener, so there's no cross-window access. Chromium browsers optimise for this and open the new page in its own process.


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