You could but where is fails is when you open one html file that then needs to loads the other files that are needed by the first.
You can not allow chain loading like this, it would bypass the sandbox.
One way of working around this would to allow the option of passing a whole folder and sub folders to the program.
The other and much harder option would be a per program portal filter that can read the html file. then workout what files that html file needs and offer that list of files to the user.
The lazy work around is allow read access to $HOME and deny access to some files and folders like .ssh