Section titles support roles just like all blocks.

This is the correct syntax. It generates the following HTML:

By default, the HTML converter wraps the section in a div. That's where the role goes. To style it, you'd use:

.category-title > h3 {

  ...
}

or

.sect2.category-title > h3 {

  ...
}

If you want to change how the HTML is generated, you need to use a custom template, in this case for section (e.g., section.html.slim). Here's an example: https://github.com/asciidoctor/asciidoctor-backends/blob/master/slim/html5/section.html.slim.

More information about custom templates can be found in the following issue: https://github.com/asciidoctor/asciidoctor.org/issues/80

Cheers,

-Dan
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