make4ht and tex4ebook support --mode option, which can influence the compilation. If user doesn't provide his own build file, a default one is used:

if mode=="draft" then
Make:htlatex{}
else
Make:htlatex{}
Make:htlatex{}
Make:htlatex{}
end

As the build file is a Lua script, I can imagine tat it shouldn't be so hard to add some function which would compute checksums for selected files, compare these with checksums from previous runs of given command (latex, biber, xindy, etc.) and run it only when checksums changed.