Technical SEO
Crawlability, page speed, structured data, canonical tags and clean internal linking. It's unglamorous, and it's the part most often skipped. A site that loads slowly or contradicts itself in its own markup will underperform, whatever is written on it.
It's also the part that decays without anyone noticing. A redesign can break canonical tags, a plugin can start producing duplicate metadata, and nothing flags it. That's why ongoing measurement matters as much as the first fix.