What is an XLSMHTML file?
A file with .xlsmhtml extension is an HTML version of the original XLS file that is exported from within Microsoft Excel to view the data in a browser. The format has long been discontinued by Excel and is no more supported. The exported XLSMHTML file followed the XLS file format and these could be opened and viewed in web browsers. Excel provides exporting the spreadsheet data to MHT and MHTML file formats, and are widely used now.
XLSMHTML File Format - More Informaiton
XLSMHTML files were generated with HTML code for representing the spreadsheet data in a similar format to XLS. The HTML code inside the XLSMHTML file is arranged in such a way that the output file looks similar to the original file when opened in a web browser. The file format specifications of XLSMHTML file are not as such made public by Excel and no such references are available for developers to understand and develop applications for it.