The .xsl View Files
Views in InfoPath are created dynamically using XSLT 1.0 stylesheets (.xsl files), with one XSLT file corresponding to each view that has been created in the form template. The Microsoft flavor of XSLT is almost 100% compliant with the XSLT 1.0 Recommendation from the W3C, so if you are familiar with any flavor of XSLT 1.0, it is unlikely that you will run into implementation-specific syntax difficulties when editing XSLT files in InfoPath.
SHOP TALK PRESERVING HAND-EDITED XSLT CODE
When you make hand edits in XSLT and later work in the InfoPath client in design mode, the edits might be overwritten by the XSLT code generator. To protect code from being overwritten, you must use a mode attribute with the value of xd:preserve ...