I am so glad I found this sticky. I have been tearing my hair out trying to find out why epubs that look perfect in Calibre and Sigil show up without any formatting at all on my PRS-T1. Thanks to the validator I found a stray } in my stylesheet.
Question: does anyone know of any issues with using font-size for the headings in the CSS? I've had an issue with the headings not showing up at all on my Reader that was resolved by removing the font-sizes I'd defined. The headings worked just fine on my computer.