Today we release MooTools Core 1.4.4 which is a critical maintenance release. 1.4.3 release introduced a bug as a result of fixing another bug. Specifically, 1.4.3 did not allow custom attributes (e.g.
data- or non-standard attributes). See this issue for a full explanation and solution.
We recommend that all users upgrade to 1.4.4 as soon as possible.