It is strange your html file is called "untitled.html"? Although legal in epub2 it is probably not a good idea in general.
Yes, epub3 requires .xhtml files not .html files otherwise epubcheck warns you.
Once you have loaded your epub into Sigil as an epub2, you can easily globally rename all .html files to be .xhtml files and Sigil will properly handle everything. Then run the epub3-itizer plugin on it and you should be good to go at least for that warning.
For details, please see the first post in this thread (where you downloaded the plugin from).
Quote:
Before Conversion
1. Make sure your epub2 validates under FlightCrew and/or EpubCheck
2. If your epub uses ".htm" or ".html" file extensions, you should first change them to use ".xhtml" file extensions as follows:
In Sigil's BookBrowser window select all of your .htm or .html files
Right click on your selection and select Rename from the context menu
in the "Rename Files Starting At" dialog box, remove everything there and replace it with ".xhtml" (no quotes), then hit "OK"
|
I am not sure what the "http-equiv" attribute error is about yet. I will look into it.
Hope this helps.