Earlier quoted context omitted.
That's true, but Safari uses a closed source JavaScript engine. Seems to me it doesn't count for much unless the whole browser is Free and Open Source. A single non-Free component is enough for them to sneak in functionality that you don't want. See also: the VSCodium editor/IDE. Official builds of Visual Studio Code do not correspond to the published Visual Studio Code source-code, as they ship with additional telem…
Safari's JS engine is open source: https://trac.webkit.org/browser/trunk/Source/JavaScriptCore
Is it used in any fully Free and Open Source browser?