Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

Admonitions are written through mdx directives. Mdx parses passed class names and provides them in the mdast:

:::info{.important}
:::

gives

{
  "type": "root",
  "children": [
    {
      "type": "containerDirective",
      "name": "info",
      "attributes": {
        "class": "important"
      },
      "children": []
    }
  ]
}

I would expect the resulting admonition to include the class name, but it doesen't.

I came accross this issue when i wanted to format md-tables inside an admonition with smaller cell padding. Instead of

:::info{.small}
| a      | markdown |
| ----- | ------------ |
| table | inside        |
:::

i needed to do

:::info
<div className="small">
| a      | markdown |
| ----- | ------------ |
| table | inside        |
</div>
:::

which works fine, but clutters the md content.

Reproducible demo

https://stackblitz.com/edit/github-on5zd6ky?file=docs%2Fintro.md

Steps to reproduce

1. yarn run start
2. navigate to the tutorial intro where the table example from above is shown, the css is inside custom.css

Expected behavior

directive classes should be propagated.

Actual behavior

directive classes are ignored

Your environment

• Public source code: https://stackblitz.com/edit/github-on5zd6ky?file=docs%2Fintro.md&file=src%2Fcss%2Fcustom.css
• Public site URL:
• Docusaurus version used:
• Environment name and version (e.g. Chrome 89, Node.js 16.4):
• Operating system and version (e.g. Ubuntu 20.04.2 LTS):

Self-service

• I'd be willing to fix this bug myself.