All XHTML components are derived from the org.zkoss.zhtml.impl.AbstractTag class.
A XHTML component is a thin wrapper that encapsulates a native HTML tag. It is different from a XUL component or other none-native component in several ways.
By implementing the org.zkoss.zk.ui.ext.RawId interface, the universal identifier (getUuid) is the same as the identifier (getId).
By implementing the org.zkoss.zk.ui.ext.DynamicAttributes interface, all XHTML components support arbitrary attributes. In other words, any attribute name is legal (as long as the targeted browser supports).