I'll give it a try.
- Opened a known good ePub. Well-formed, epubcheck and epubveri showed no issues.
- Ran Mend and Prettify. Pretty Printing windows popped up and ran.
- Removed DOCTYPE from 1 file. Mend and Prettify popped up the error window listing that file. See attached image.
- Removed DOCTYPE from 2 more files. The error window mentioned the first file. The popup was identical.
- Ran Mend. The 3 missing DOCTYPEs were restore and M&P ran.
Well-formed check, epubcheck and epubveri did not show any errors for the missing DOCTYPE which is correct since ePub3 does not require the DOCTYPE. Ghod alone knows where Sigil is picking it up as an error.
Hmmm... latest code from GitHub, Python 3.14.6, Qt 6.10.2, Microsoft (R) C/C++ Optimizing Compiler Version 19.51.36252 for x64