DOCTYPEBUG
X:.
│   Demo.py
│
└───static
    │   index.html
    │   indexfail.html
    │
    └───css
            style.css
The only difference between index.html and indexfail.html is the first line.
If is omitted then the background looks fine in green and we have a pleasing Hello world
If it is included then css won't be used for display.
Tested with Cherrypy 18.1.2 on Windows