Describe the bug
When enabling the attr_list extension:
https://squidfunk.github.io/mkdocs-material/setup/extensions/python-markdown/#attribute-lists
https://python-markdown.github.io/extensions/attr_list/
the following anchor link is valid:

![My image](image.png){ #my-image-anchor }

Blah blah [my image reference](#my-image-anchor)

but mkdocs-htmlproofer-plugin will report it as invalid url:

invalid url - #my-image-anchor [404] [mypage.md]

Contents of mkdocs.yml:

markdown_extensions:
  - attr_list

plugins:
  - search
  - htmlproofer:
      validate_external_urls: False