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.
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.