There is only one file type that uses the .XLTHTML extension:
Microsoft Excel HTML Spreadsheet Template
An XLTHTML file is an HTML version of an .XLT spreadsheet template created with Microsoft Excel, a spreadsheet application. It contains HTML that renders the template using similar formatting as the original XLT file. XLTHTML files are used for viewing published spreadsheet information in a Web browser.
Microsoft Excel no longer supports the XLTHTML format. Instead of saving documents as XLTHTML files, Excel supports saving them as .MHTML and .MHT files, which are more widely supported by web browsers.
To create a webpage version of your template in Excel 365, select, then choose the "Single File Web Page (*.mht, *.mhtml)" option from the dropdown menu. You can also choose to save the template as an .HTM or .HTML file.
Unfortunately, there are no known applications that fully support the XLTHTML format since Microsoft Excel dropped support for it.
Updated: May 22, 2021