To summarize the many good responses I've gotten (thanks!), which have helped my mental model a ton: On the supportive side: Embedding arbitrary code (mostly,

To summarize the many good responses I've gotten (thanks!), which have helped my mental model a ton: On the supportive side: Embedding arbitrary code (mostly, but not entirely in browsers) with secure sandboxing. On the skeptical side: Just a way to make the browser less of a user agent and less accessible by running opaque code blobs.