in reply to Similarities of Perl and Python?
I would wave in the operating system knowledge.
HTML and other web browser technologies are on the application (browser) level. Not much os knowledge is needed there, but at least some basic understanding of browsers and client operating systems is beneficial, if not necessary.
As for the back end though, that can be close to servers' os. So depending on the os you're having in use or plan to use in the future (which you don't mention), I'd do some screening in those areas. And here comes the C level very likely. Very likely, because C programming doesn't necessarily have to be system programming, and the same applies to Python. They both can be used productively without much concern to the executing os, especially Python. For Unix-like based back end, I'd definitely check the ability of working with standard documentation, the awareness of POSIX, skills in basic system operations. I'd begin with questions: what did you program in C and script in Python and shell? Which shell was that and why? How much interaction with the os did it involve? Do you think it was a good tool for what you did? Did you miss some functionality? What would make it better, easier, more appropriate for the task? About C: what libraries did you use and why? What compiler? This should give you an overview of who you're dealing with.