I'm beginning to work on a project which has some extensive XML XSLT processing to render output HTML.
Some changes need to be made to the XSLT and I need some tool that can help me modify it without having to run the solution every time. Something that can help me visualize the changes I'm making to the rendered HTML.
I've found StylusStudio but I preferably would want a freeware that I could use
It's not freeware, but Altova XMLSPY is pretty powerful XML IDE. It offers an XSLT debugger where you can step through your conversion, as well as generate output(HTML in your case) from a sample XML document with the XSLT document you are working on.
CoherentWeb is a commercial tool designed to do this. Its not an XSLT development tool, its for the batch-testing, publishing and reviewing of XSLT 1.0/2.0 output and is specially suited to XHTML.
This is a .Net tool with a graphical UI and is multi-threaded for performance/resilience. Its totally focussed on input/output and can handle a single HTML file or thousands in a folder hierarchy (like, for a website). It rebuilds your folder structure in a clone, so that all relative links can be tested, there are also watch-variables you can set up to monitor HTML meta-data etc in your output.
You just select a top-level folder, select the XSLT and run. You can quickly review rendered input and output for comparison, selecting an input file automatically shows you the associated output, an XML report summarises all XSLT outputs. Settings are stored so you don't have to repeat things each time you start up the app and want to re-run the XSLT.
This is so different from a typical IDE that its hard to visualise, but there is a YouTube video demo of a website transform if you're interested.