I've finally took a look at this issue, sorry for the delay. There are two issues. The easier one can be fixed in the TeX4ht code. There was just wrong CSS instruction that caused drawing of the line over the whole row that contains the \cmidrule. It should be fixed in the sources now. The fix is also in the attached .cfg file.

The secong issue is worse - it seems that each \cmidrule inserts separate row, so even if the lines have correct width, they are not aligned vertically. I've created Lua filter that should join rows created from one \cmidrule into one.

It seems to work on your sample document.

(file #331, file #332)

The attached tex file, and html generated by ` make4ht -uf html5+tidy ht-tabular-book.tex`, illustrates some issues with \cmidrule from booktabs.

(I did not use the build.lua file from bug #441, to avoid complicating matters, but it does not affect this bug.)

The first table is 6 columns wide and contains a single \cmidrule{2-3}. There are two problems:
(1) the cmidrule should be over columns 2-3, but it is over columns 1-3 instead.
(2) the HTML is not valid -- the cmidrule produces a tr containing a td spanning 1 column and a td spanning 2 columns. But the table is 6 columns wide.

I note that 'rule' effect is achieved with a CSS border applied to the tr, not the relevant td, which might explain (1).

The second table contains multiple non-overlapping \cmidrules, not an uncommon use-case for booktabs, which should appear side by side. In addition to the problems above, each \cmidrule gets its own tr, which means that they appear stacked on top of each other.

Finally - this may be more of a feature request than a bug, and it applies to the other rules too - is it the right thing semantically to have separate tr to put rules in? For instance, you would not want a screenreader to read out an extra row in this situation.