We had tried to implement mermaid.js into Marp Core once, but we've stopped working because of some technical problems.
yhatt/marp#125 (comment)

In Marp Core, you can try to use markdown-it plugin for enhancing features as you like.
https://www.npmjs.com/search?q=keywords%3Amarkdown-it-plugin%20mermaid

And you can also use mermaid.js in Marp through inline script (Require to enable insecure HTML in your tool): yhatt/marp#125 (comment)

In VS Code for Windows, I managed to get Mermaid working in the preview by using a div tag. (It doesn't seem to render with ```mermaid). There are no obvious problems. But it doesn't appear in the HTML or PDF export. I'm wondering if there's a quick fix for this.

The <div> tag workaround doesn't seem to be working well on my side, the font size is too big to fit in the boxes...

Not quite sure why it's working but here is my workaround anyways:
Adding a custom theme (mermaid.css) that imports your favourite theme and changes the font-size of the mermaid class:

/* @theme mermaid */

@import "uncover";

.mermaid {
  font-size: 10px;
}

Then, when creating the div add the following:

<div class="mermaid">
%%{init: {'themeVariables': { 'fontSize': '200%'}}}%%
flowchart LR;
    A([Some node])-->B;
    B[Some other node]-->C;
    B-->D;
    C-->D;
</div>

where the fontSize should be some value >= 200%, also allowing you to scale your charts.
Leaving me with:

Of course you also need to set the theme accordingly and add your custom theme as described here.

I've made the basic plugin example to render various diagrams at
https://gist.github.com/yhatt/8931dd98769bcc1c5b4b92fd1234a190. It is powered by kroki.io for reliable rendering in Marp slide.

Currently we have no plan to integrate as the default feature of Marp to prevent wasting kroki.io instance by free riding.

Cycling back to the using the <div> trick: is there a way to make it render in the HTML output? I don't know much about these things but it seems logical that if we do get a nice rendering in the marp preview we should get one also in HTML 🤔 Am I missing the point here?

Aren't you using Markdown Preview Mermaid Support extension in VS Code? This extension will try to recognize <div class="mermaid"> and render the diagram in the preview of VS Code, if Marp preview mode was enabled or not.

VS Code has no ability to get "exactly" same rendering result as built-in Markdown preview if modified preview pane by scripts provided from other extensions. Thus, the export HTML (provided by internal Marp CLI) may be differ from the preview.

Again, as described in yhatt/marp#125 (comment), just adding 2 lines at the end of Markdown is working for me.

<script src="https://unpkg.com/mermaid@8.1.0/dist/mermaid.min.js"></script>
<script>mermaid.initialize({startOnLoad:true});</script>

These scripts manually instruct rendering mermaid diagrams on behalf of Markdown Preview Mermaid Support extension.

Ability of extending Marp export process by VS Code extensions has been discussed at marp-team/marp#176.

Maybe this could be a way to add mermaid to marp. https://github.com/mermaid-js/mermaid-cli#transform-a-markdown-file-with-mermaid-diagrams. When there would be a way to add preprocessors to the engine, the marmaid-cli could transform the mermaid code to SVG before marp is rendering it.

In the meentime I am using the kroki-plugin. However kroki has at the moment a mermaid font rendering problem...

@yhatt Unfortunately it doesn't work for Mac.

I use the mermaid plugin, and it works for .md files but when I use it for marp, it doesn't recognize it. This is what I get it.

@tisuchi Marp for VS Code does not allow raw HTML tags by default because of security reason. You must opt-in to use HTML tags manually.
https://github.com/marp-team/marp-vscode#enable-html-in-marp-markdown-%EF%B8%8F

After enabling HTML from the preference, you can use mermaid with <script> tag. Probably you may also have to mitigate the security setting for VS Code's Markdown preview.

---
marp: true
---

<div class="mermaid">
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;
</div>

<!-- Put this script at the end of Markdown file. -->
<script type="module">
import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@10/dist/mermaid.esm.min.mjs';
mermaid.initialize({ startOnLoad: true });

window.addEventListener('vscode.markdown.updateContent', function() { mermaid.init() });
</script>

Thanks for your answer @yhatt

I found the same solution and I note it here for further reference for others.

https://laravel-school.com/snippet/how-to-enable-mermaid-or-html-directive-with-marp-in-vs-code-62

using the div tag for mermaid ，but it doesn't support for gantt picture？any way to support？

@cmi1993 Try using <pre> instead of <div>.

---
marp: true
style: pre.mermaid { all: unset; }
---

<pre class="mermaid">
gantt
    title A Gantt Diagram
    dateFormat  YYYY-MM-DD
    section Section
    A task           :a1, 2014-01-01, 30d
    Another task     :after a1  , 20d
    section Another
    Task in sec      :2015-01-12  , 12d
    another task      : 24d
</pre>

<script type="module">
import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@10.0.0/dist/mermaid.esm.min.mjs';
mermaid.initialize({ startOnLoad: true });

window.addEventListener('vscode.markdown.updateContent', function() { mermaid.init() });
</script>

it works, thank you very much! amazing!

---
marp: true

---

<pre class="mermaid">
gantt
%%{init: {'theme':'neutral'}}%% 
        dateFormat  YYYY-MM-DD
        axisFormat %m-%d
        title xxxx
        excludes    weekends
        
        section xxxx
        xxxx           :active,lxian1 ,2023-03-04,2023-03-31
        xxxx     :zhaobiao1,after lxian1, 45d
        xxxxx	        :crit, milestone,zb1, 2023-05-01,2023-05-01
        xxxx  :fangan1,2023-05-01,25d
        xxxxx        :devTest1, after fangan1,12w
        xxxz	    :crit,milestone,duiba,2023-07-10,2023-07-10
        yyyy          :yansou1, after devTest1, 22d
        zzzz            :  after yansou1,22d
        ddddd	     :crit,milestone,2023-12-10,2023-12-10
        
        
        
        section xxxx 
        xxxx           :active,lxian, 2023-03-04,2023-03-31
        xxxx       :crit,poc,2023-03-31, 30d
        xxxx    :   zhaobiao,after poc, 45d
        xxxx	        :crit, milestone,zb, 2023-06-10,2023-06-10
        
        xxxxxxxxxxxx  :    fangan, after zb,25d
        xxxx         :     devTest, after fangan,12w
        xxxx(07-10)	    :crit,milestone,duiba,2023-07-10,2023-07-10
        xxxx         :   yansou, after devTest, 22d
        xxxx            : 22d
        xxxx(12-10)	     :crit,milestone,2023-12-10,2023-12-10
           
</pre>

<script type="module">
import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@10.0.0/dist/mermaid.esm.min.mjs';
mermaid.initialize({ startOnLoad: true });
window.addEventListener('vscode.markdown.updateContent', function() { mermaid.init() });
</script>

I have been successfully rendering gantt pic by using Marp and

tag ，I the pic is not centering，how to fix it? thanks alot

Would be great if we could get native support for this in marp(mermaid: true), I use mermaidjs extensively in the README.md files in my projects on GitLab.

Guys, for the time being i've managed to set up a boilerplate that allows for

1. previewing Marp and Mermaid in Vscode's markdown side preview
2. creating pdfs that are consistent with the previewed code
3. having text don't cram up in Mermaid's boxes

You can check it out here iacobucci/mermarpid :)

I'd like to mention a slightly different approach to make sure that mermaid diagrams are rendered correctly.

I noticed that Mermaid diagrams rendered to actual svg files actually render nicely in Marp if you add them as images. That's because the generated <img> tag properly isolates the svg from the slides' CSS and document structure.

So a simple workaround to correctly render mermaid in Marp is to:

• Collect all mermaid diagrams in the slides
• Have Mermaid render them to svg fragments
• Encode the svg fragments to base64
• Replace the diagram nodes with <img> tags sourced with the encoded svg

Code:

---
theme: white
---

<!-- Use marp --html --watch <this file> to compile  -->

<style>
  img.mermaid-100h {
    max-height: 100%;
  }
</style>

<script src="https://unpkg.com/mermaid/dist/mermaid.min.js"></script>
<script>
  // Replaces <pre class="mermaid"> blocks with <img> blocks, to make mermaid render properly.
  // Preserves classes and styling so they can be used to fix sizing if necessary.

  mermaid.initialize({ startOnLoad: false });

  window.addEventListener('load', async function () {
    const mermaidEls = document.querySelectorAll('pre.mermaid');

    for (const el of mermaidEls) {
      const { svg } = await mermaid.render('asd', el.textContent);

      const img = document.createElement('img');
      img.setAttribute('src', `data:image/svg+xml;base64,${btoa(svg)}`);
      img.setAttribute('class', el.getAttribute('class'));
      img.setAttribute('style', el.getAttribute('style') || '');

      el.parentNode.replaceChild(img, el);
    }
  });
</script>


### Flowchart test

<pre class="mermaid mermaid-100h">
flowchart LR
A --> B --> C
lorem1[Lorem ipsum 1] --> lorem2[Lorem ipsum 2]
</pre>