Sorry, it is mentioned in the help so I didn't think it would need much detail.

https://www.w3.org/wiki/Graceful_degradation_versus_progressive_enhancement#Graceful_degradation_and_progressive_enhancement_in_a_nutshell

se doc -h includes "If a proper JavaScript file is not given with the -js flag, the 'navigation' will not be possible (except for browser which are not CSS compliant)."

In any browser that doesn't support JavaScript, or has it disabled, but does support CSS then the documentation is mostly useless without user intervention to disable or alter the site's CSS.

E.g., http://doc.liberty-eiffel.org/api/libraries/index.html only displays a list of clusters with non-functioning links and provides no access to the classes because they're hidden with CSS.

For browsing documentation I primarily use either dillo or lynx, neither of which support JS.  It works in lynx because it doesn't support CSS but not in dillo which does support CSS so I have to disable the style sheet for it to work.

Also, when using Firefox I use the popular NoScript addon and having to add a whitelist exception or user style sheet seems unnecessary.

I've never come across any other API documentation that requires JS, CSS or both for basic functionality.  It doesn't seem desirable and would appear to only require a simple change to default to showing the classes when JS is disabled.