"the file you are trying to open is in a different format from the one specified by the extension and may be corrupted??"
I don't get that error, it opens without saying anything when I try it. Still it might be that your excel is more sensitive when it comes to contents that aren't exactly excel data.
All the html would be extra stuff.
Its good for presenting it in a webpage, but do you really need all that behind a spreadsheet? It might be just me, but i really see no need for all the html there in the actual excel file. I don't know if I'm making any sense.
Yes I understand the user saves the web page and then opens it in excel, but i don't see the use of all the html inside the spreadsheet. I would probably just copy the information into the spreadsheet rather than opening the html file.
Still it works and can be done, and f you need the niceties, like bolding and colors then yes it is useful.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.