<img> also took me the longest time, and <br> even longer though I used it within the last hour, but the facepalm moment came when I almost decided to give up a

<img> also took me the longest time, and <br> even longer though I used it within the last hour, but the facepalm moment came when I almost decided to give up and then remembered <h1> through <h6>. I even knew how many levels existed! Not that I didn't try h7 for good measure, one can't be too sure with these things :-)