On previous posts [^][^] I mentioned the size of ASP.NET validators and validation summary rendering and the fact that expando attributes are being used to add properties. Mohamed also mentions this issue.
Besides the fact that custom attributes aren't XHTML conformant, Firefox differs from Internet Explorer in the way it handles these attributes.
On Internet Explorer, these attributes are converted in string properties of the HTML element. On Firefox, on the other hand, these attributes are only accessible through the attributes collection.