A file with .xlthtml extension is an HTML version of XLT template files and were generated using Microsoft Excel in the early days. This format is no more supported by Excel and has been replaced by exporting data to MHT and MHTML file formats. The basic idea behind the XLTHTML file was to enable users to view the XLT template files in a web browser instead of the need of having Excel installed on the system. Files exported in XLTHTML file format could be opened in browsers.
XLTHTML File Format - More Information
XLTHTML files used the HTML file format to save the XLT templates for opening in a web browser. XLT itself is based on the Excel binary file format that was introduced with Excel 97-2003 version. These files can be used as starting files with defined formatting to create normal documents for population of spreadsheet data. Just for reference, XLT is based on the binary XLS file format specifications which is documented by Microsoft for developer’s reference.