I can't find anything in the EPUB spec (although I'm not familiar with it, so that's based just on some skimming and Ctrl+F-ing) that would suggest going against letter and spirit of the HTML5 spec like that.
It sounds like Sigil is merely implementing a workaround against an internal bug (
source):
Quote:
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- Update sigil_bs4 to use numeric entities when faced with nbsp so they do not get lost later in Sigil
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Calibre says:
Quote:
HELP = _('Named entities are often only incompletely supported by various book reading software.'
' Therefore, it is best to not use them, replacing them with the actual characters they'
' represent. This can be done automatically.')
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That's true, but there's a slight difference between the basic XML named entities (<, >, &, " and ', without which a program can be considered severely defective), HTML basics like and — which are universally supported, and more exotic ones like, I don't know, bigodot; or something. :P