This extension for Visual Studio Code allows you to run XSLT transformations with the editor. It utilizes the open source xslt-processor JavaScript module from Fidus Writer.
You can install this extension from the Visual Studio Code Marketplace.
While editing an XML file, hit Ctrl + Shift + p to bring up the command palette, and type XSLT in the palette window. Select "Run XSLT Transformation" from the options. You will then be presented with a dialog to pick an XSLT file. Choose the XSLT file and the transformation will run. Two windows will then open to the side: One showing the transformed text and one attempting to display the browser rendering of that transformation (in the case of HTML output).
This extension is barebones, and just performs transformations. There's a lot more it could be built out to do. Feel free to request functionality via GitHub issues.