How can I setup a group writeable directory?
First, enable the SGID bit on your directory:
sudo chmod g+s html
This will make new files created inside that directory inherit the parent's group ownership.
There is no inheritance of permission levels in the POSIX permission model. However, this can be done with Access Control Lists, without having to worry about umask settings:
sudo setfacl -d -m group:web:rwx html
It's a real bummer that umask cannot be assigned on a per-directory basis.