At exactly 400px width, everything disappears

[dracos]

Both the display:nones match, so you can't see anything. (Chrome's default smallest-width appears to be 400px on this computer which is why I spotted this when trying to view the small-width behaviour.)

[Heydon]

@dracos Weird. It doesn't do this to me (Chrome/OSX)

[dracos]

Sorry, should have included screen shot:

Changing the min-width to 401px (or the max-width to 399px conversely) would fix it.