As explained in https://bugs.documentfoundation.org/show_bug.cgi?id=162186, opening an HTML with three to four é consecutive characters in LibreOffice opens it as the HTML source code, but without these characters. This happens because the libwpd library reports excellent confidence that the file is supported (happens in WPDocument::isFileFormatSupported, src/lib/WPDocument.cpp).
The attachment is a sample of such file.
https://sourceforge.net/p/libwpd/code/ci/bc3050ad804b4f9f60ccfaf425f469fc095033dc/
https://sourceforge.net/p/libwpd/code/ci/bc3050ad804b4f9f60ccfaf425f469fc095033dc/