Memo comments can be put in an HTML file within the code itself without altering the appearance of the web page rendered by the HTML file, but I don't believe that's what you have in mind here.
I believe you want to do is analogous to taking a photograph and annotating notes on the face of the photo. If that's correct, one idea is to do a print screen, i.e. screen capture, of the web page as it appears when viewed with a web browser and then open the image, i.e. screen print, in a photo editing program and annotate notes on that image. Even the simple Microsoft Paint program could be used for this, but more sophisticated photo editing programs might work better for this.
