> And I can't test the new version of Safari on my Windows or Linux devices because Safari isn't available for them.

You can download three Linux-compatible WebKit-based browsers from Apple's WebKit page:

Epiphany Technology Preview: https://webkitgtk.org/epiphany-tech-preview

WPE: http://wpewebkit.org/download

WebKitGTK: http://webkitgtk.org/download