I am suspecting the html file is corrupt or it is not ok. I compared with another file that I downloaded after writting in the online p5 editor, and a key difference lies in the fact that the empty example has the script tag that calls the sketch.js inside the head, whereas in the working file the script tag is inside the body.
Try placing the script tag in the body.
Also the css styles have a canvas selector with a display property set to block.
it could also be a problem with the local server program. IDK.