If you are using the Oxygen editor, you can debug Toolkit transforms using the technique described here (from the Oxygen 14.2 documentation):
Debugging PDF Transformations
To debug a DITA PDF transformation scenario using the XSLT Debugger follow these steps: Go to Options > Preferences > XML > XML Catalog, click Add and select the file located at [Oxygen Install Directory]\ frameworks\dita\DITA-OT\plugins\org.dita.pdf2\cfg\catalog.xml;
Open the map in the DITA Maps Manager and create a DITA Map PDF transformation scenario;
Edit the scenario, go to the Parameters tab and change the value of the clean.temp parameter to no;
Run the transformation scenario;
Open in Oxygen XML the stage1.xml file located in the temporary directory and format and indent it;
Create a transformation scenario for this XML file by associating the topic2fo_shell.xsl stylesheet located at OXYGEN_INSTALL_DIR/frameworks/dita/DITA-OT/plugins/org.dita.pdf2/xsl/fo/topic2fo_shell_fop.xsl;
In the transformation scenario edit the Parameters list and set the parameter locale with the value en_GB and the parameter customizationDir.url to point either to your customization directory or to the default DITA OT customization directory. It's value should have an URL syntax like:file://c:/path/to/OXYGEN_INSTALL_DIR/frameworks/dita/DITA-OT/plugins/org.dita.pdf2/cfg.
Debug the transformation scenario.
I found this topic by searching for "debug toolkit" in the Oxygen online help.
These instructions are specifically for PDF, but you should be able to adapt these instructions to HTML-based transforms as well.