I am currently working on an annotation tool for an EPUB reader. The reader is written in Typescript and uses node.js and the Readium Desktop toolkit. My approach saves an annotation.xml file in a dedicated annotation folder within the epub, reference it in the publication's manifest and .opf file and inserts each annotation while loading (in the preload file of the readers webview).
The issue I'm facing is that while adding another annotation and therefore forcing a reload of the page I'm getting an Invalid local header error along with Asset not declared in publications resources/spine: EPUB/undefined/readium-css/ReadiumCSS-after.css which is as I understand the reader's default css. But then again I do not understand why the reader would look for that in the EPUB file.
This error only occurs on "newer" EPUBs such as [https://www.kobo.com/at/en/ebook/accessible-epub-3] but not on "older" ones like some I got from [https://www.gutenberg.org].
I am a bit unsure what code snippets would help at the moment so feel free to ask for more details.
I would really appreciate any help.