Implements methods for working with HTML attributes, parsing and generating attribute strings. Port of HTML_Common class for PHP4 originally written by Adam Daniel with contributions from numerous other developers.
Called if trying to change an attribute with name in $watchedAttributes
This method is called for each attribute whose name is in the $watchedAttributes array and which is being changed by setAttribute(), setAttributes() or mergeAttributes() or removed via removeAttribute(). Note that the operation for the attribute is not carried on after calling this method, it is the responsibility of this method to change or remove (or not) the attribute.
Attribute value, null if attribute is being removed