After Responsive Image went into core (back then it was still called "Picture"), the spec for the tag continuously evolved. For example, the sizes and type attributes ware added to the source tag, which wasn't supported in the original Responsive Image module. So in Responsive image module does not support sizes/picture polyfill 2.2 we did the following:
-Update picturefill library to latest version.
-Make sure picture and source tags are correct.
-Remove the setting of width and height on source elements.
-Add type support for source tag (code, not UI).
-Allow source tag without a media attribute (code, not UI).
-Add support for sizes on source tag.
-Update UI of responsive images mapping.
-Add work around for IE9, http://scottjehl.github.io/picturefill/#ie9.
Besides that we did some major refactoring of the code and wrote a lot of documentation for all the existing and new functions, classes and methods, so that the code became much clearer for developers that are not necessarily familiar with the picture spec.