I'm not a big fan of questions that provide some criteria without reasoning for a solution to be acceptable but in this case, I have one too: No JavaScript for compatibility, except Modernizr. (The reason is that they slow the page down too much.)
I used the new and fancy CSS3 flex-box to create a menu, which displays some extra links, if there is some extra space. You can see a (hopefully) working demo here.
The big question is: How can I provide a fallback for the (cough)annoying(/cough) Internet Explorer and the old browsers without using the somewhat famous FlexieJS library? Is this achievable with just css2, other than setting a maximum percentage width for the menu (ul)s (which does not really work)? Could you at least point me in the right direction? Thanks!
UPDATE
I gave another try but still couldn't figure it out on my own. Any help is really appreciated! Thanks a lot =) You can find my attempt here.