Strict HTML strips CSS classes in a custom block I created a custom block using Strict HTML (and adding &lt;div&gt; and other tags to the allowed HTML). If I look at the frontend, it strips all the CSS classes like &lt;div class="myclass"&gt;.

What do I have to do to create this block with all the CSS classes, and the custom HTML markup?
Default custom blocks in Drupal 8 use the Basic HTML text format. Using either Basic HTML or Restricted HTML will remove any class attributes. 

You need to set your format to Full HTML to prevent Drupal from stripping out these attributes.