Technical review: Document nested pseudo-elements

[chrisdavidmills]

Description

Chrome has added support for styling nested pseudo-elements, beginning in version 135 with ::after::marker and ::before::marker. See https://chromestatus.com/feature/5199947786616832.

This PR:

• Adds a section to the main pseudo-elements reference page that explains nested pseudo-elements and provides a place to list all the supported ones.
• Adds a basic demo to the ::before and ::after pages to show the nested pseudo-elements in action.

Note: Unless I've missed something, there doesn't seem to be a really obvious use case for these two nested pseudos — it seems like they are just part of setting up the system for more useful things to come in the future, like carousel/fragmentation (see https://github.com/flackr/carousel/tree/main/fragmentation)

Motivation

Additional details

Related issues and pull requests

[github-actions]

Preview URLs

• /en-US/docs/Web/CSS/::after
• /en-US/docs/Web/CSS/::before
• /en-US/docs/Web/CSS/Pseudo-elements

Flaws (2)

Note! 1 document with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/CSS/::after
Title: ::after
Flaw count: 1

• macros:
  • Macro produces link /en-US/docs/Glossary/replaced_elements which is a redirect

---

URL: /en-US/docs/Web/CSS/::before
Title: ::before
Flaw count: 1

• macros:
  • Macro produces link /en-US/docs/Glossary/replaced_elements which is a redirect