non-(X)HTML extension => file copy to TMP folder ... this breaks relative URLs to CSS, images, etc.

This works for unit-tests, because the HTML markup does not refer to satellite resources (which will not be the case for real-world documents):

https://github.com/daisy/ace/blob/be5d8a4154a2a38a07b28475f9e0538d91a0bab3/packages/ace-core/src/checker/checker-chromium.js#L33-L42

...therefore, this seems like a broken/useless logic, right?

Unit tests:

https://github.com/daisy/ace/blob/be5d8a4154a2a38a07b28475f9e0538d91a0bab3/tests/__tests__/regression.test.js#L71-L74

https://github.com/daisy/ace/blob/master/tests/data/issue-122/EPUB/content_001.xml
https://github.com/daisy/ace/blob/master/tests/data/issue-122/EPUB/content_002.ace

Start with the non-(X)HTML handling in packages/ace-core/src/checker/checker-chromium.js at the referenced lines, then run the regression coverage in tests/__tests__/regression.test.js and inspect the issue-122 fixture files. Reproduce a document whose CSS or image uses a relative URL and verify that checking a non-(X)HTML extension preserves access to its satellite resources.

Written by the indexing model from the issue text.