Continuing the discussion from What is your current project?:
You don't need to resize the window to see the responsive layouts. Both Firefox and Chrome have responsive tools you can use to view the current page in a number of configurations. You can access these panels by hitting 'f12'.
Here is a few things I noticed, that you could consider adjusting.
You do not test for HTML 5 support. This may cause issues in older browsers (specifically older IEs) where they do not recognize (and thus ignore) some of the newer HTML 5 tags. If this is a specific use case for you, you'll need to come up with a way to test for and adjust the site to support the older browsers while still offering the correct tags for the newer ones.
You are loading 3 different CSS files. These can be combined / minified (remove whitespace) via tooling to reduce the number of round trip HTTP requests, and the size of those requests. You can drop the type attribute on these as well, as they are not required in HTML 5 spec.
You load the jQuery library early on in the head, without needing it till just before the body closes. You can move this lower in the page, as where it stands now, it's blocking. Blocking is when the client stops rendering to parse the JS.
Hiding the H1 tag falls in the 'no' side of Google's Webmaster Guidelines. You may want to review / adjust for this.
Consider removing some of the responsive code from the JS, you should be able to handle some of this with CSS and Media Query Breakpoints. Specifically, the menu div could be placed by default and hidden. You could choose to display #menu and #nav based on breakpoints, rather than adding and removing classes manually and doing manual content injection. The $('#nav').removeAttr() code never seems to fire as you would expect, because as near as I can tell, #nav never has a style attribute to remove (the code works, but it removes something which isn't there to begin with).
This may be just me, but I also found the font in the 'mobile' menu to be overly large.
If you intend the site to be shared, you should look into Open Graph Meta tags.
Since you are using static HTML, this site may run into maintainability issues (adjust the menu on one page, have to replicate the code on every page).
I only recommend this when you are getting ready to launch, but you can improve load time by setting cache headers on your content.
Overall however I think you are off to a good start. Responsive layouts from scratch can be tricky and yours seems to be working well. Keep up the good work, and feel free to ask questions on any of the above if you have them.