Catskull’s I’m betting on HTML has a list of interesting HTML elements. I summarize a few I found of note.
<details>
<summary>click to see</summary>
...rest of message
</details>
This text appears <mark>highlighted</mark>.
<input type='color|date|range'>
<meter id="fuel"
min="0" max="100"
low="33" high="66" optimum="80"
value="50">
at 50/100
</meter>