(Rereading again..) The JS claim is silly. He is saying that because we are dynamically creating a single style, that show/hide logic will be faster. I disagree with this, since adding a CSS selector to the page means the page needs to rematch that new selector against every element of the page and re-render each matching element, same as if we had just changed the matching elements with javascript.

EDIT: thinking on this some more, it actually should be a bit faster for huge collections, since javascript has to update items individually, whereas the browser has probably optimized this code path.

Well, it's fewer DOM manipulations, right? Touch a single style element vs updating a bunch of them.

It's fewer DOM manipulations from Javascript yes. The browser will still internally change the styles associated with the DOM elements, just without the JS overhead.