From https://bugzilla.mozilla.org/show_bug.cgi?id=233474 The text at bug URL uses a Windows-only trick to display mathematical symbols: <i>A</i> <font face="symbol">Ì</font> <i>B</i> Where Ì is an 8-bit code for "SUBSET OF" in this font. I'm not sure if this problem deserves being fixed, but it is clearly a problem.
<rdar://problem/3248204>
A lot of MS-Word-exported html files have this problem as well: They have : <font face="webdings">abc 123</font> : to get some dingbats <font face="Symbol">S</font> : to get Greek Capital Sigma
“Symbol” does not support the codepoints associated with the latin alphabet.
The Mozilla bug is WONTFIX, and I agree with that assessment.