The original Markdown spec is very clear that HTML is allowed. So MD itself is absolutely a superset of HTML https://daringfireball.net/projects/markdown/synta

The original Markdown spec is very clear that HTML is allowed. So MD itself is absolutely a superset of HTML https://daringfireball.net/projects/markdown/syntax#html https://daringfireball.net/projects/markdown/syntax#html But in practice people mostly use MD variants, such as the "GitHub Flavored Markdown Spec" which may have some limits on HTML usage https://github.github.com/gfm/#html-blocks https://github.github.com/gfm/#html-blocks