As far as I am aware, the current gold standard for checking epubs for error is epubcheck. The requirement for .xhtml as the file extension was dropped by epubcheck several versions back and should no longer cause a warning message.
As for renaming the files, you could use Sigil to select all the .html files in the book browser and then rename them to *.xhtml. I think the calibre ebook-editor has the same functionality but I've never used it for that purpose.
Last edited by DNSB; 06-03-2023 at 03:54 PM.
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