<t a='b'c='d''e'="f">
is clearyly a syntax error. If you interpret 'd''e' as the attribute value of c, then you have a spurious = after it, which is not allowed. If you interpret 'd' as the attribute value if c, then you have a quoted attribute name, which is not allowed.
Yes, I know, the question is if changing that is perhaps more dangerous than changing how the quoting works.
A real html compatible parser would be very nice when parsing actual HTML, but perhaps not 100% compatible with RXML (one somewhat common usecase of Parser.HTML, although HTML parsing is probably more common.)