You can't just insert HTML content into a cell and expect Excel to render it, Excel doesn't natively support HTML in it's cells. Even using the
IsRichText
doesn't work because it expects RTF-formatted content.
How can I embed HTML file in excel? - Stack Overflow[
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Some suggestions include instead embedding a browser viewport within a cell and then rendering it there. I'm not sure if this works, you might need to search around.