html5 code in sidebar block

Not sure if the “problem” is with the template or with how blocks work, so apologies in advance …

I added some basic html5 code in a block

<p>Below you find definitions of the music styles, taken from Wikipedia in April 2018.</p>

<aside>
<details>
<summary>Goth metal</summary>
<p>Gothic metal (or goth metal) is a fusion genre combining the heaviness of heavy metal with the dark atmospheres of gothic rock. The music of gothic metal is diverse with bands known to adopt the gothic approach to different styles of heavy metal music. The genre originated during the early 1990s in the United Kingdom originally as an outgrowth of death-doom, a fusion of death metal and doom metal. Lyrics are generally dark and introspective with inspiration from gothic fiction as well as personal experiences.</p>
</details>
</aside>

And this works nicely in my HTML5 template if I insert it in an article. However, if I add this in a block and put the block in a sidebar – the code isn’t rendered. It looks like a block is supposed to be able to take this content, right? Or am I doing it wrong (again?).

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/22/feed