what the conformant applications should do in the presence of behavioral markup. There are no required actions, only recommended ones: show: new, replace, embed, other, and none actuate: onLoad, onRequest, other, and none Semantic attributes are xlink:title, xlink:role, and xlink:arcrole. The title attribute can appear on any element to provide a brief description. The role attribute can appear on extended, simple, locator, and resource ele- ments. The arcrole attribute can appear on simple and arc elements. Both xlink:role and xlink:arcrole must be absolute URI, perhaps with a fragment identifier attached. Their intended use is rather vaguely defined, but see the fol- lowing RDDL section for an example. In addition to the xlink:title attribute, extended link-, locator-, and arc- elements can have any number of children elements of type title, that is, ele- ments with xlink:type=title. Their purpose is to provide a more structured and extensive annotation or a series of annotations, perhaps in different lan- guages. (The value of an attribute can only be CDATA.) Summary of XLink Attributes Table 2-2, quoted from the XLink specification, shows all XLink attributes and how they coexist with different values of xlink:type, listed as column headers. Now that you have seen them all, this table may actually be useful. (R stands for required, O stands for optional, and the N/A stands for not applicable.) Table 2-2. Summary of XLink Attributes ATTRIBUTE SIMPLE EXTENDED LOCATOR ARC RESOURCE TITLE type R R R R R R href O N/A R N/A N/A N/A role O O O N/A O N/A arcrole O N/A N/A O N/A N/A title O O O O O N/A show O N/A N/A O N/A N/A actuate O N/A N/A O N/A N/A label N/A N/A O N/A O N/A from N/A N/A N/A O N/A N/A to N/A N/A N/A O N/A N/A 64 Chapter 2