Markdown makes it easy to write formatted content at the cost of initial compiler setup, HTML is 0-cost setup but takes some effort to write formatted content. With a <markdown> tag one could start with vanilla HTML at zero cost, use all its features straight away, but not have to type HTML for the simple, "content" parts of a site.
EDIT: there seem to be web components for this already:
- https://github.com/xan105/web-component-markdown
- https://www.webcomponents.org/element/zero-md
- https://www.webcomponents.org/element/@vanillawc/wc-markdown