Amazon Send-to-Kindle service has accepted EPUB, however, for historical reason it still assumes ISO-8859-1 encoding if no encoding is specified. This creates weird formatting errors for special characters.
This tool will try to fix your EPUB by adding the UTF-8 specification to your EPUB. It currently does not fix other errors.
You can access this tools at https://kindle-epub-fix.netlify.app
Warning: This tool come at no warranty. Please still keep your original EPUB. We don't guarantee the resulting file will be valid EPUB file, but it should.