PS ---
Calibre and Sigil are based on QT, which honors css rather differently than ADE.
Many reading device softwares are based on the ADE RMDSK, so you might consider installing one or two versions of ADE For Desktop on your computer for testing purposes.
Version 2.0.1 is older, but still handy, version 3 changed some of the css it will honor.
https://www.adobe.com/support/digita...downloads.html
When you have tested with both QT and ADE rendering, your book stands a better chance of good display in a wider range of devices.