Answered responsive menu like the one IBM is using

Yes as always iex is a pain in the ...
and yes graceful degration is the way to go. And its easy to communicate to! Just tell the customer the aproximate additional cost for supporting ie8 or even below *wimper and they usually spontaneously decide that these oldtimers are not to be bothered with anymore

You see, i have a markup first principle. My solution stack looks like:
Solve everything with the markup -> solve the remaining with css -> solve the remaining with javascript


So it would be nice if the submenus of the menu implementation would be a normal member of the site flow(so no position or floating business) and would so push the following content down by itself.

Again, great conceptual workflow. The main issue you're going to run into is that the functionality you want pretty much requires floats and positioning. You could use display:inline-block instead of floats, but that leaves a small gap that must then be accounted for in your CSS and causes layout issues as a result.

No-one disables CSS, so solving layout using markup (instead of just making sure it is semantic) is contrary to the intended roles of HTML and CSS.

Perhaps your workflow needs to be slightly modified to read like this:

1. Write clean, semantic markup.
2. Solve any positioning, design or layout problems using CSS.
3. Solve any remaining issues using JavaScript.

Just my two cents!