fs module not found

Well, fs is too much for the ESP8266.

• Use the Storage module to save pages and css (store_webdata.js)
  
• Load them in your app (in_your_app.js).
  
  

store_webdata.js

require("Storage").write("mypage", `
   your page data
`);
require("Storage").write("mycss", `
   your css data
`);

in_your_app.js

mypage = require('Storage').read('mypage');
mycss =  require('Storage').read('mycss');

On ESP8266 Storage is located in the memory mapped section, using this technic is fast and saves vars and heap.