We are in the process of migrating and upgrading a client's SharePoint 2010 Intranet portal to SharePoint 2013. Our client has many custom web parts built in XSLT. These are basically Content Query Web Part running with custom XSL code. Would these web parts run after migration/upgrade?
Even the XSLT feature deprecated in SharePoint 2013 but still has Backward compatibility. But your testing will prove it.