More testing reveals that the font rendering problem (blank blocks) is only presented in RN preview mode and not when the exported html is opened with Chromium or Firefox. I think you should take a look at the rendering engine.
Also, as far as the font substitution concerns, DejaVu Sans is an adequate substitution for Ubuntu whenever there is a missing character as the two are almost indistinguishable at a glance. Therefore, this font line should do nicely: "font-family: Ubuntu, DejaVu Sans, Arial, sans-serif;".
More testing reveals that the font rendering problem (blank blocks) is only presented in RN preview mode and not when the exported html is opened with Chromium or Firefox. I think you should take a look at the rendering engine.
Also, as far as the font substitution concerns, DejaVu Sans is an adequate substitution for Ubuntu whenever there is a missing character as the two are almost indistinguishable at a glance. Therefore, this font line should do nicely: "font-family: Ubuntu, DejaVu Sans, Arial, sans-serif;".