As folks said, cap resis FIRST, then build into armour applies to ele hits after. Armour applies to ele IS important - in addition to the detailed math explanation in other thread, when you start running maps you’ll notice mods with “monsters deals extra % of damage as fire/lightning/cold” - these are relatively “smaller” hits but deadly nonetheless.
Having amour applies to ele significantly reduces such hits (again since those are smaller portion of, say, a monster that originally does phys dmg only) and drastically boosts survivability.
You can also look into “damage taken as fire/lightning etc” as a further layer of mitigation and make you super tanky.
Carnarius (YouTuber) show cases a build with only armour + ~2400 life tanking the most dangerous hits at end game, you can check it out.
TLDR Armour applies to Ele is VERY important, especially at end game. Start stacking after you hit 75 ele res