[millstone]

Banker vs moon rover. B&D languages panic at the first whiff of deviation between the compile-time and runtime environments. But I loaded cnn.com, and my third-party ad blocker did god-knows-what to dozens of requests, with a bazillion unexpected undefineds, and yet the page still rendered.

The web powers through!

[lomnakkus]

This has nothing to do with undefined vs. exceptions, sorry. You can do example the same with a powerful enough sandbox -- e.g. what WebAsm does to avoid hostile code taking over your computer.

It has everything to do with sandboxing (or not). Ads being served by third parties should not be able to influence the JS running in the page.