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That wouldn't work, because jQuery would have multiple versions in the wild, custom builds, forks, etc.. Not only would almost no one have a cache hit because of that, but every time a CDN is down or overloaded, your website is now artificially slow... or broken, which happens far more often than a cache hit.

Even if you have it cached I think that the parse tree is not, so the whole thing must be processed again and again.

That only applies if you serve from a public CDN, which there are reasons to not want to do.

Including jQuery in browsers is kind of a ridiculous idea...