One of the biggest technical SEO mistakes I still see developers making is treating SEO like a “post-launch task” instead of part of the architecture from day one.
You already mentioned semantic HTML and Core Web Vitals — both are huge — but I’d add poor crawlability as another major issue. A lot of modern sites rely heavily on JavaScript frameworks without thinking about how search engines actually render pages. If the content, metadata, or internal links depend entirely on client-side rendering, indexing problems usually follow.
Another common one is ignoring proper internal linking and heading structure during development. I’ve seen beautifully designed websites where every section is just nested divs with no meaningful hierarchy at all. Looks great visually, but search engines lose context fast.
Personally, I think developers who understand even basic technical SEO have a huge advantage today. It’s much easier to build performance, accessibility, and crawlability into the project early than trying to “SEO-fix” everything later.
At Apklounge, I’ve been experimenting with balancing fast-loading pages and SEO-friendly structure for gaming-related content, and even small optimizations to image delivery and page structure can noticeably improve visibility.