Zepto is already a popular option for those who want jQuery features in a lightweight library. The API is essentially identical to jQuery, making it a drop-in replacement at a fraction of the size.
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?
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.
That doesn't list IE as a supported platform at all (not even the current IE9), so while you should be OK (if IE is now as close to compliant as MS claim it should just work) if you find problems in IE you might be on your own when it comes to researching and fixing them.
See http://zeptojs.com/#platforms - specifically "we may add support for Internet Explorer 10 ... depending ..." and the lack of any mention of IE before 10.
zeptojs.com