Some specification UI recomendations

[jiezhang917]

https://html.spec.whatwg.org/multipage/forms.html#the-textarea-element

First of all, thanks for such a detailed document to help me learn knowledge on HTML standards. I just feel like if you can include the following tips it would be better for general readers:

1. A float content list on the side of the window, so that we can go to any tips we are interested in or want to review again.
2. A 'top' button may makes the page more user-friendly if it's too long.