Creating a customization plugin
The steps required to create a customization plugin does not differ from others DITA-OT plugin.
Please read fist the DITA Open Toolkit Developer Reference.
Ant import extension point
The ant target dita2d4p-html5 has the dita:extension org.dita.dost.platform.InsertDependsAction
XSLT import extension point
The XSL template map2html5_template.xsl has the extension point id xsl.transtype-html5