Support for dark mode alternate images in markdown

[ybizeul]

Feature Description

GitHub supports embedding both variant of an image in markdown (See https://www.stefanjudis.com/notes/how-to-define-dark-light-mode-images-in-github-markdown/)

Basically it works by using this syntax :

![fperm schema](./schema-dark.svg#gh-dark-mode-only)
![fperm schema](./schema-light.svg#gh-light-mode-only)

It would be nice to have this supported in Gitea markdown

Currently, both images are displayed.

Screenshots

[silverwind]

According to https://github.com/stefanjudis/github-light-dark-image-example, that hash-based syntax is "deprecated" and the <picture> syntax should be preferred. Does it work in Gitea?

<picture>
  <source media="(prefers-color-scheme: dark)" srcset="./dark.svg">
  <img src="./light.svg">
</picture>

We should still support the GitHub sytnax for compatibility reasons of course.

[wxiaoguang]

Gitea doesn't support <source> at the moment.

It should be an easy fix to support <source media...., just add them to the func (st *Sanitizer) createDefaultPolicy()

[ybizeul]

According to https://github.com/stefanjudis/github-light-dark-image-example, that hash-based syntax is "deprecated" and the <picture> syntax should be preferred. Does it work in Gitea?

We should still support the GitHub sytnax for compatibility reasons of course.

I didn't know about deprecation, well that's too bad but indeed mirroring GitHub behavior is usually a plus.

That said, the options listed below in your example page work better as they fall back on light image instead of displaying both, that's a win.

So I replaced :

![fperm schema](./schema-dark.svg#gh-dark-mode-only)
![fperm schema](./schema-light.svg#gh-light-mode-only)

with:

<picture>
  <source media="(prefers-color-scheme: dark)" srcset="./schema-dark.svg">
  <img alt="fperm schema" src="./schema-light.svg">
</picture>

As @wxiaoguang said, it doesn't work in Gitea indeed, and only light image is displayed

[silverwind]

Yeah we will need to adapt the markdown sanitizer for that to work. After that, we can also try to support the hash syntax.