JavaScript rendering
JavaScript rendering is the execution of client-side code by the engine to see the final page. Google does this on a deferred basis; other crawlers and most AI data collectors do not execute JavaScript at all.
In practice
The practical consequence: all decisive content must exist in the initial HTML. Applications rendered entirely client-side are invisible to a large part of the engine ecosystem and to nearly all AI assistants. Server-side rendering or static generation resolves this at the root — it is the default choice on every site we build.