Sigil has never validated to device specific limitations.
MRSDK barfs on
allowed codes that it is supposed to ignore.
I can't fault it for intolerance to malformed code
(Sigil/Calibre tolerate some errors, thus adding to
when it hits the device and fails..)
The above malformed will almost always result in the CSS being
ignored by MRSDK (a clue)
CSS quick proof read
a Attribute never has a space (page-break-after)
values with units have no space (5em)