Earlier quoted context omitted.
>If only jQuery was modular and didn't require to load 300 or 400 Kb of code. JQuery's own site says the library is 30 to 40 Kb, not 300 or 400. The minified JS file I downloaded of the latest version is 87 Kb, so I assume the rest would be taken care of with zip headers. Are people routinely serving the uncompressed version of the library?
Agree. Plus one can use a cdn to increase the chances of the library being already downloaded.
Browser caches are partitioned by origin so if a.com and b.com both use jquery from c.com then two copies of jquery will be cached one with a key of a.com/c.com and another with b.com/c.com