Markdown is created by Daring Fireball, the original guideline is here. Its syntax, however, varies between different parsers or editors.
Please note that HTML fragments in markdown source will be recognized but not parsed or rendered. Also, there may be small reformatting on the original markdown source code after saving.
Hugo loves Markdown because of its simple content format, but there are times when Markdown falls short. Often, content authors are forced to add raw HTML (e.g., video <iframes>) to Markdown content. We think this contradicts the beautiful simplicity of Markdown’s syntax.
Hugo created shortcodes to circumvent these limitations.
A shortcode is a simple snippet inside a content file that Hugo will render using a predefined template. Note that shortcodes will not work in template files. If you need the type of drop-in functionality that shortcodes provide but in a template, you most likely want a [partial template][partials] instead.
In addition to cleaner Markdown, shortcodes can be updated any time to reflect new classes, techniques, or standards. At the point of site generation, Hugo shortcodes will easily merge in your changes. You avoid a possibly complicated search and replace operation.
More details: https://gohugo.io/content-management/shortcodes/