### Bug description

I've been working on a presentation at my job with a `http://myserver.local:8000/` being on the local network. The presentation includes Mermaid diagrams, and when the demo URL isn't available (whilst at home), the diagrams don't render.

My workspace is a Docker container with Quarto cloned, in previewing mode, with Vim as editor (no plugin installed).

**Environment**:

* Host MacOS `13.3.1` (x86)
* Docker `python:3.8` (Debian I believe)
* `CMD ["quarto", "preview", "/Content", "--port", "8000", "--host", "0.0.0.0", "--timeout", "0", "--no-browser"]`
* `jupyter matplotlib plotly` through pip

**Quarto check**:

```bash
$ docker exec keynote-presentation-1 quarto check

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.1: OK
      Dart Sass version 1.55.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      Path: /Service/package/dist/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.8.16
      Path: /usr/local/bin/python3
      Jupyter: 5.3.0
      Kernels: python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........(None)

      Unable to locate an installed version of R.
      Install R from https://cloud.r-project.org/
```

**Minimum working example**:

`_quarto.yml`:

```yml
project:
  output-dir: _output
  render:
    - index.qmd
format: revealjs
title: "Presentation"
view-distance: 3
```

`index.qmd`:

````md
---
title: "foo"
---

## My Mermaid slide

```{mermaid}
flowchart LR
  A --> B
  B --> A
```

## My img slide

<img src="http://myunreachableserver.local:8000/">
````

Commenting out the `<img>` tag will render the diagram instead of it echoing its declaration.

### Checklist

- [X] Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
- [X] Please [format your issue](https://quarto.org/bug-reports.html#formatting-make-githubs-markdown-work-for-us) so it is easier for us to read the bug report.
- [ ] Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
- [X] Please document the operating system you're running. If on Linux, please provide the specific distribution.
- [X] Please provide the output of `quarto check` so we know which version of quarto and its dependencies you're running.
2023-05-02T19:48:25Z
QSmally
closed
Mermaid diagrams not rendering when unrelated `<img>` tag has unknown host
https://github.com/quarto-dev/quarto-cli/issues/5387