It's really interesting ReactOS can run both Chrome and Firefox. The latest versions of these browsers do not run on any old Windows: Chrome has required at least Windows 7 since version 50 (April 2016) and Firefox has required this since version 53 (April 2017) - although Firefox 52 ESR is still supported until May 2018.
Although I don't follow ReactOS, I thought they were working on supporting older Windows (Wikipedia says they try to run on Windows Server 2003 drivers). So does this mean they are catching up so fast to newer Windows versions or are we in fact seeing (very) old browsers in these screenshots?
I don't think the ReactOS people are trying to do Windows versions like Russian nesting dolls. They try to aim to fully support the older version, but while they're trying to fill all the nooks and crannies of them, quite often you'll find yourself adding some new API from a more recent version, if that's all you need to support a specific kind of application.
I'd naively assume that finishing the undocumented API calls of the last century is a much more time-consuming task than adding what little came after 2003...
I'm not seeing the claimed browser support. The ReactOS Application Store has Firefox 45, which successfully updated to Firefox 47... but then Firefox 52 ESR crashes on start. I'll have to uninstall and go back to 47 to get Firefox to work.
The Application Store has Opera 12.18 (from 2012), so I used that to try and download Chrome. Chrome tells me my computer isn't supported, but offers a download anyway. But when I try it, the installer stops after "On your marks!" so I have no idea how they took that Chrome screenshot.
While they target an older driver API, the userspace is mostly Wine's, so it is much easier for them to support recent software.
In fact, for the last 10-15 years, Wine itself to some extent is written against Windows kernel APIs such as ntdll.dll, and only the lowest levels talk to the host OS and wineserver. This means that ReactOS can keep also some of Wine's lower level DLLs, such as kernel32.dll, as long as there is something below them.
Although I don't follow ReactOS, I thought they were working on supporting older Windows (Wikipedia says they try to run on Windows Server 2003 drivers). So does this mean they are catching up so fast to newer Windows versions or are we in fact seeing (very) old browsers in these screenshots?