If I may, I think this issue should be extended to really support unknown elements for IE7/8. The problem is that the current implementation has ''a list of unknown'' elements.
This causes in my view (at least) two problems:
1. If tomorrow html5 decide to add a new element, old versions of the applications that use the current jQuery version will suddenly stop working until they upgrade their jQuery, and for any new element this list must be upgraded.
2. This approach also doesn't allow custom elements to be created that are not (and will never be) part of HTML5. We are actually using this in an application, where we create custom rendered metadata nodes on the fly (we receive them via AJAX). Elements are things like <loop>, <metatarget>, etc.
I am currently working on a fork where I try to allow creation of completely unknown elements, because currently (in the 1.7 beta):
has a different outcome in IE8 than in IE9 (or any other decent browser).
I hope you find these requirements acceptable for reopening this, or even better in my opinion reopening the duplicate issue 10427. (since HTML5 support is implemented, and only custom elements are not working).