i18n & l10n
How to design a web app so it can be translated, localized for currencies and dates, and laid out for right-to-left languages — without rewriting it later. ICU MessageFormat, locale routing, the timezone trap, and the right way to do plurals.
SEO
The technical-SEO essentials a developer actually owns — meta tags, OpenGraph, JSON-LD structured data, sitemap.xml, robots.txt, canonical URLs, and the Core-Web-Vitals/SEO connection.