For people developing for newer browsers only (e.g. browser extensions or corporate), I'm curious to know how many of the guidelines still apply. e.g. are forEa

For people developing for newer browsers only (e.g. browser extensions or corporate), I'm curious to know how many of the guidelines still apply. e.g. are forEach/map slow or are the anonymous functions inlined? Also, I'm surprised that he says that setting style properties one at a time is slow; I would have thought that reflow happens lazily (at the next read of a layout property or on the next idle spot on the event loop). My main takeaway was that HTMLCollections (including the return value of getElementsByTagName, form.elements, etc.) are evil. Looking at the jsdom source, the one thing that really bugs me is that they have to maintain all these live lists on every change.