You can do the same in Asciidoc with the passthrough feature, either as an inline passthrough or a passthrough block. Granted, it's not syntax-less like Markdow

You can do the same in Asciidoc with the passthrough feature, either as an inline passthrough or a passthrough block. Granted, it's not syntax-less like Markdown, but it's there if you need it. Inline example: pass:[<u>underlined</u>] or +++<u>underlined</u>+++