You will need to use the FireFox browser. Start it and type into the address bar: about:config. Then hit enter key. Then click on 'Accept the Risk and Continue'. Then type privacy.file_unique_origin and set it to false. Then type security.fileuri.strict_origin_policy and also set it to false. Then close FireFox and start it new so that the setting takes affect. Now open any of the old HTML ebook files with the browser. You do this by clicking on the three bars icon in the upper right corner of the browser. Then look for the "Open File ..." menu item and select the HTML file you want to view.