As a frequent keyboard user, after writing a comment in the comments box, I pressed TAB on my keyboard to focus on the "Submit" button. I didn't know if I was focused on it because it seems that there is no outline when focusing on that button (or the cancel button next to it).
However, the "Write a comment" button above it has an outline and doesn't have this problem.
I also fired up a screen reader out of curiosity and I also noticed a potential issue in the rich editor. Many of the buttons such as Write, Preview, Guide and Markdown are audible with a screen reader. However, some of the buttons such as Bold and Italics aren't. The screen reader just reads "clickable button", instead of "button bold" or something relevant.
Additionally, the buttons in the rich editor don't have outlines either. This means that they don't seem usable with a keyboard or a screen reader.
Please reply or contact me if you have any questions or feedback.