There are many methods that break performance and accessibility such as dynamic css classes and div and span soup.

Topics adressed from the outline:

1. It’s not just bad HTML – it’s meaningless markup
2. Semantic rot wrecks performance
   1. Big DOMs are slow to render
   2. Complex trees cause layout thrashing
   3. Redundant CSS increases recalculation cost
   4. Autogenerated classes break caching and targeting
   5. Animations and the compositing catastrophe (with properties triggering the layout engine)
   6. Autogenerated classes break caching and targeting
3. Semantic tags can provide layout hints
4. (AI) Agents are the new users and they care about structure
5. Structure is resilience, isn't optional

Likewise, content-visibility: auto is one of the most underrated tools in the modern CSS arsenal. It lets the browser skip rendering elements
that aren’t visible on-screen – effectively “virtualising” them. That’s huge for long pages, feeds, or infinite scroll components.

and I didn't know about the contain CSS property.

contain: layout; tells the browser it doesn’t need to recalculate layout outside the element.
will-change: transform; hints that a compositing layer is needed.
isolation: isolate; and contain: paint; can help prevent visual spillover and force GPU layers.