According to Max's CL comments:

"""
I don't have a strong opinion here, but have a few suggestions. We can discuss it and decide later, current implementation looks good.

1) index.html file shows per directory coverage, but also has a link at the top, something like "Click here to see coverage report for all source files" (needs a better phrasing I guess)

2) index.html file shows per source file coverage, but has a link at the top, something like "Click here to see per directory coverage". I like this less than the 1st, as coverage per directory seems to be more useful in a general case

3) index.html file has two links to different pages: to per directory coverage and to per file
"""

It needs more investigations to understand what's best to display in the index.html file of coverage report for better user experience.