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> Is this the right place for dropdown-submenu? I'd have expected it to be on the <ul> below..

Doesn't seem to work on the `<ul>` below, I was just looking @ the example on stackoverflow and I did it how they had it.  I'm not really that knowledgeable about CSS.

> Is the purpose of the new CSS to fix it on mobile? It seems to make no difference, so I suspect it's not being used correctly.

The new CSS restores the submenu option, it's not there in bootstrap anymore because it doesn't work on mobile (no way to hover).  Did you remove the `<link>` tag? Maybe your browser still had the dropdown.css cached?