Finally, some mechanism should be introduced to allow control over
content in the HTML header. In particular, some means is needed to
introduce style sheets.
I don't know about style sheets, but some form of html output control
may be desirable. On the other hand, ultimate flexibility in output is
not what Texinfo is about -- you can't do much at all to control the way
the TeX output appears either. It's more for common input than perfect
output.
You may want to look into texi2html, it already does have some output
control. http://texinfo.org/texi2html/
