Could you expand on that at all? I realise that Zepto covers a (very) small subset of jQuery functions, but I thought that in a lot of use cases it would be an appropriate substitute?

It doesn't normalize DOM events, which is a big one for many people.

Last I checked zepto doesn't have support for any of the simulated selectors like `:checked`, `:input` and friends. If you use any of them, its not drop in.

If you build zepto with experimental CSS selectors you can. https://github.com/madrobby/zepto/blob/master/src/selector.j...