It seems to me that there is a difference between savetrees, which simply makes no sense in HTML, and tcolorbox, which does. I don't think Michal was advocating blanket application of blacklisting packages.
fontspec is another case where it is clear that whatever it does in the TeX world, it has to do something completely different in the HTML world. Anyway, fonts are a difficult issue. As for authors fixing packages to work with tex4ht, does that ever happen :(? In any case, a warning and log message about non-inclusion make sense to me, so the non-inclusion is at least visible, not completely silent. (Aside from absence of reading the savetrees.sty file ...) -k
