You can blame who you want, but it isn't "ExtJs' fault". It's Microsoft for refusing to comply with open standards. And that's ignoring the fact that MS only recently killed support for a 9 year old browser version. There are plenty of things that ExtJs could do better (CompositeFields being a container, RowEditor actually working etc...). But at the end of the day the craptastic browser that is IE is due to Microsoft's unwillingness to use open standards and (perhaps more importantly) the fragmentation they cause by refusing to end-of-life products that (by web standards) are ancient.