Why does <!docType HTML> render as text/plain, in a .QMD file?

[beedell-roke]

Description

When I render discussions/14935#discussioncomment-18568454 to /HTML inside quarto preview $HOME/qmd.qmd --no-browser --no-watch-inputs, I observe that it is visible, which it surely should not be, as invisible HTML, in any CommonMark superset, should be invisible when rendered.

[mcanouil]

Could you make your question self-contained, with a small reproducible example?

The comment you link has no Quarto document in it, but your command renders qmd.qmd.
Please share that file, or a cut-down version that still shows the problem, along with your Quarto version.
I'm also not sure what you mean by "to /HTML".

Also, Quarto isn't a CommonMark superset, so that assumption won't hold.
Quarto 1 uses Pandoc's Markdown, which has its own rules for detecting raw HTML.
To avoid detection issues, put your HTML in a raw block, as already described in https://github.com/orgs/quarto-dev/discussions/14935:

```{=html}
<!DOCTYPE html>
...
```

Note that because of the partials/templates for HTML, all the HTML you placed in the body of a Quarto document won't end up in the correct place, they will all be inside <body>.