None of those errors are stylesheet errors
Code:
'sigil_toc_id_10': fragment identifier is not defined in 'OEBPS/Text/Chapter 7.xhtml'
Smashwords has been reported to be overly fussy.
The error syntax context is iffy as to which end the mismatch is Chapter 7.xhtm
Chapter 7.xhtm <<
Never use spaces in file names. you will not regret it later. Use of an underscore is acceptable for readability.
if you use digits, use leading 0's to pad to the number of highest digits so a file sort (if needed) puts most into order. Chapter_07.xhtm
the first part of the error says there is a
reference (
somewhere) to the anchor
id="sigil_toc_id_10" or "../Text/Chapter 7.xhtml#sigil_toc_id_10" (To be honest, I have never seen Sigil construct this usage variation). The former are usually inserted into <H# tags as part of generating a NCX (IIRC the HTML TOC builds off of the current NCX,
NOT the document
The fact that Flight crew does not throw an error makes me think this is Smashwords being fussy.
Have you Validated using the W3C site ??