[DiapDealer]

There's scads of threads documenting how the "start reading" functionality of a Kindlebook doesn't always seem to work as expected. Bottom line: it's not a Sigil (or even an ePub) related issue.

My best advice would be to forget about setting a "start reading" point. Even if you get it to work after conversion with Kindlegen/Previewer, there's no guarantee it won't be over-ridden by Amazon's KDP process.

If you CAN'T forget about it (or a client won't let you), I suggest searching for the various threads about kindle- mobi-books and setting the Start Reading point... and/or taking it up with Amazon.

EDIT: also... validating all your markup BEFORE you create your epub is all well and good, but it's sort of pointless if you don't then validate your epub after you're done with it. Valid (x)HTML != Valid ePub.

If you want to set the start reading point with semantics, use the 'Text' semantic, not the 'Title page' one.