I know XSLT in general can be debugged, but how exactly would one go about debugging a DITA transformation, considering its modular XSLT structure and the fact that stylesheets are pointed to by the catalog.xml file?
I want to be able to step through the code during runtime, and be able to set break points, etc.
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.