Since at least v18, InDesign export to HTML writes any user-designated CSS files ahead of the base, idGenerated CSS file. This means user statements are wholly or partially overwritten by the base statements.

This has been discussed and noted in the forum since at least v18, and I thought a bug notice had been entered here but I cannot locate it.

This fault is critical for any users trying to export to HTML and use CSS style overrrides. Problems are likely elusive and frustrating since the "wrong interaction" of the CSS order may not be apparent unless and until the user examines the export header code.

The only workaround is to open every export file and manually reorder the CSS import statements.