When I have needed it, I have used ABC Amber's CHM Converter. (OK, I have used a lot of ABC Amber's products and found them to be great value for the money.) Their web site is here.
It will produce a single HTML file output as well as many other formats. It is free to try and if you want to purchase it the price is $19.95 for a single copy (cheaper if purchased as part of a bundle. I have many of their products -- both free and for fee -- and have found them extremely useful.
I have no relation to the company other than as a customer for several years.