I just caught on that you have to send it to make it work. This is a start, thanks for video, it gave me the clue that I needed.
No, you don't have to send it to "make it work." Instead, there is the
preview button, and when you press it, the preview appears above the place where you are creating your reply and you can scroll up to see exactly the way your reply will look. You can iteratively tweak your reply and press the preview button. Once you are finished editing your reply to make it look the way you want, then you can finally press the
post button.
Also, once you have pressed the button to post your reply, if you spot an adjustment to the way your reply looks that you are just dying to make, you (as the original author) can make that change (although there is no preview for that, but you can keep going back and making changes until you're completely satisfied). The "modify" icon of the yellow pencil applied to the paper (

) that appears above your reply (and is also at the end of your reply) is what you press in order to apply such later changes.