A few people asked if HTML Spell Checker can check XML and XSLT files. Not directly, as XML files are handled by a separate XML Editor which my add-in does not directly work with. However, you do can spell check XML and other markup based files if you open them as HTML since HTML editor is able to parse XML files. Here is how to do it:

1. Right-click on the file in the Solution Explorer and choose 'Open With...'.
2. Select HTML Editor and click Open.

If file is not in the solution:

1. Choose File | Open
2. Click on the small arrow on the Open button and select Open With...
3. Select HTML Editor and click Open.

Now Tools | HTML Spell Checker command should become available.

You may have to customize spell checking rules in the rules.xml file (see the main post) as the file is tailored to HTML and ASP.NET files.