These days using "nav" instead of <div class="nav"> is the preferred method by default. The other two classes are just modifiers that may or may not be required

These days using "nav" instead of <div class="nav"> is the preferred method by default. The other two classes are just modifiers that may or may not be required. There's nothing wrong with them. Also, I see "fixed" bug reports in both Chrome and Firefox when using flex with fieldset. To be fair, recent fixes.