I fully agree with TedPride. Put the global (site-generic) CSS and JavaScript code into seperate files (which can be cached), and inline the dynamically created (page specific) CSS and JavaScript parts directly into your HTML file. By inlining them you avoid extra HTTP requests. There remains only an increase of the HTML file size which should not make such a big difference.

Concerning run-time, dynamically creating CSS/JavaScript does not make much sense. Because of potential caching, it would be better to write a lot of page-specific .css and .js files (put aside the extra HTTP requests).

However, thinking about generating and - more importantly - maintaining the .css and .js files, it can be a huge benefit for the web developer to use dynamically created CSS/JavaScript. Often a simple Perl script can serve as the generator for a wealth of page specific codes. Any changes can be made in this single script. On the other hand, changing dozens of different .css and .js files can be a tedious and error-prone task.

The maintaince aspect is the very reason why I am sometimes using dynamically generated CSS/JavaScript. I think it is a useful technique, and practice didn't prove me wrong, so far.