SVG content type missing entry for class-name

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/SVG/Content_type

### What specific section or headline is this issue about?

class-name

### What information was incorrect, unhelpful, or incomplete?

an entry for class-name is missing

### What did you expect to see?

This page should include an entry for class-name.
Note that w3c says that a class name should follow the css syntax rules.
However I notice a complete different behaviour of the browser:
- only accepts digits, hyphen, underscore, and case sensitive ascii letters (<0x7f)
- can't start with digit or hypen+digit or hypen+hyphen
- does not accept escaped characters
- accepts a single character (even a single underscore character)

on https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/class
The link on the value item should link to that new class-name entry.

### Do you have any supporting links, references, or citations?

_No response_

### Do you have anything more you want to share?

_No response_

### MDN metadata

Page report details

* Folder: `en-us/web/svg/content_type`
* MDN URL: https://developer.mozilla.org/en-US/docs/Web/SVG/Content_type
* GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/svg/content_type/index.md
* Last commit: https://github.com/mdn/content/commit/e2fd94be1e307d18e5e2c0eadc6bd7d95f40b29c
* Document last modified: 2022-05-01T06:27:50.000Z