The only program that can reliable work and export IDML format would be the very same software that uses it, Adobe InDesign. Document prepared using this software can be exported to several publishing formats, even HTML format.
Convert InDesign Markup Language document to HyperText Markup Language web page.
We have found 2 software records in our database eligible for .idml to .html file format conversion.
Microsoft Windows software - convert idml to html on Windows
Professional desktop document publishing software from Adobe
Convert from idml file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned idml to html. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert idml to html on OS X
Mac version od Adobe's popular document designing and publishing software