I started by creating a html file on my PC to be uploaded later to the ESP8266 with some shorter html code snippets to be included. Later these snippets would contain the life data. What I found, though, is that when testing the file on the PC the include did not work. Not sure why, I could not see anything on the console of my browser.
Then I found a similar approach in https://github.com/LexmarkWeb/csi.js which worked for me right away. Just copy the short file csi.min.js into the same directory on the PC and later together with the html file into the SPIFFS filesystem. Makes creating webpages on the ESP8266 a lot easier!
PS: Chrome does not load the local page for security reasons.