I understand this, even though the constant breaking of GTK apps after each update annoys me to no end.

But you might want to discuss updating https://www.gtk.org/ and other sites for that matter. Quoting the first lines of the home page: "GTK+, or the GIMP Toolkit, is a multi-platform toolkit for creating graphical user interfaces".

Just a side note in case some representative happens to be around, the https link above delivers an invalid certificate.

And no content. I get a web server directory listing, with no entries.

The content only seems to be served via http.

If your platform is X11, Wayland, or Mir :)

And then you go to the Features page, with a box on the right clearly listing "GNU/Linux and Unix / Windows / Mac OS X" as supported plartforms :) With "Native look and feel" in the box just below.

I disagree with the current GTK direction, but it's not my project and I accept that. If the focus of the project has changed, update the website(s) and documentation.

Also the HTTPS stuff pointed out by distances.

Agreed. It seems wildly out of date. I'll track down who has access to the site and see if we can update it during the hackfest in a couple weeks.

Benjamin Otte recently added uxtheme.dll support to the CSS engine and I've already seen a few screenshots of the win32 theme using it to okay effect. I wrote a patch to add OS X cursor support to GDK last year and it was taken, not to mention work by Christian Hergert and others to improve the relatively new GDK/Quartz backend. So I'm pretty sure Windows and OS X are supported platforms :)