> the idea that one can look at a modern browser, which are some of the most complex software packages being developed, and think "clearly these people don't know how to add an 'origin' column to a SQLite database", well, it boggles the mind

It boggles my mind too. Imagine, what would have happened, if those small Javascript snippets, used mainly to add cute visual effects to pages, could check if some image from different site is already in browser cache by performing cross-site HTTP requests... That would allow completely new dimension of spying on web users!

Fortunately, browser developers are some of the most competent people in the word. They would never give web pages too much power by letting them start CPU threads, use OpenGL, allocate arbitrary amount of memory or read your battery level to set exorbitant taxi tariffs for people in a pinch. Browsers are well-designed and highly secure, because they are being updated with security fixes every day, sometimes even multiple times a day.