libxslt

libxslt is the library implementing XSLT on top of libxml2

the xsltproc program allows to run it on the command line

The API is relatively simple:

• Loading/compiling an XSLT stylesheet
• Applying a stylesheet to a parsed tree
• Saving the result according to a stylesheet

Should be compliant with XSLT-1.0 implement some 1.1 extensions

Relatively fast as long as one doesn't swap