Hello LianTze, > (Note that if \index{láboris} is changed to \index{laboris} then this can be > compiled with tex4book + xindy perfectly.) >
indexing system provided by indexing4ht is a bit fragile. It is better to use the native indexing support provided by TeX4ht. You don't need to load indexing4ht at all, you just need to use one of supported indexing commands (makeindex, xindy, xindex) in the .mk4 file. See https://www.kodymirus.cz/make4ht/make4ht-doc.html#x1-300007.1 for the available options. It is unfortunately not possible to use them from Latexmk, because these commands need to preprocess the .idx and .ind files. I can compile your file correctly using the following build file: Make:latexmk {} Make:xindex {} Make:htlatex {} Best regards, Michal