There appears to be another issue with Sigil 0.9 for plugins that import CSS files. After tidying an ePub file using the plugin at
https://www.mobileread.com/forums/sho...d.php?t=264378 a CSS file could be imported under Sigil 0.87 without any issues. Under Sigil 0.9 I find that the same plugin will corrupt the eBook - I get the dialog box that states "The operation you requested cannot be performed because Section0001.xhtml is not a well-formed XML document....". I am using the same ebook to carry out this test with the option "use bundled Python" unchecked.