Is array serialization always bad idea?
If you actually plan to query these attributes separately (such as "Find all form elements with height = 20 and colour = #FF3377") then it is probably better to store them in some kind of queryable attributes table. If you are only plan to use a set of attributes as a whole, and never query inside it, never update/insert/delete individual attributes for specific form elements, then serializing the array and storing it as a BLOB (or something) might not be so bad, as it would keep your design simpler.