Windows XP, Windows CE, and KDE Linux are specially made to be
lightweight, they take up very little space on the hard drive.
However, Windows CE is unsupported and not really common
anymore.
Can you run product development on all operating system?
No. It may be formatted different on certain operating systems
and only the operating systems running off of the central operating
system that it was built on will support it.
XHTML support is based on the browser being run, not the
underlying operating system. Any operating system capable of
running a modern browser can thus use XHTML. There are dozens of
systems that would meet this definition.