CRAN release?
I recommend that we release this package to CRAN as early as possible, because rmarkdown depends on it. CRAN will only build binaries for the R versions x.y, devel, and x.(y-1), so if we don't release htmldeps early enough, it will essentially make rmarkdown depend on a rather recent version of R. For example, if htmldeps is released after R 3.6 is out, CRAN won't build its binaries for R <= 3.4, so rmarkdown will require R >= 3.5.
BTW, if htmldeps is not on CRAN, there is no way to update rmarkdown on CRAN, either.
Yihui, as mentioned in https://github.com/rstudio/rmarkdown/pull/1390#issuecomment-432740283, there will be a PR for rmarkdown that will put the htmldeps dependencies back into that package.