Markdownlint should check for 404 links

[ViktorHofer]

see https://github.com/dotnet/runtime/actions/workflows/markdownlint.yml?page=15

2025-08-22T15:39:39.3170759Z run npm fund for details
2025-08-22T15:39:45.6510580Z Post job cleanup.

This implies that it either runs for 6 seconds and doesn't find report anything back (crash?) or that it doesn't run.

cc @Youssef1313 (as you added it years ago :)), @vcsjones

[vcsjones]

I think it is running, just that the only rule enabled is "no trailing spaces"

See

https://github.com/dotnet/runtime/blob/main/.markdownlint.json

[Youssef1313]

Yeah I think it's only that a single rule is enabled today.

[vcsjones]

Tested that it is catching the one rule we have enabled, so it looks like it's just a matter of enabling more rules, if you want them.

https://github.com/dotnet/runtime/actions/runs/18098538479/job/51495251432?pr=120197

[ViktorHofer]

Sweet. I think we initially enabled the linter to catch dead end links. Maybe we disabled that rule since then?

This came up here: #120002 (comment)

[Youssef1313]

Markdownlint isn't a markdown link checker AFAIK.

[Youssef1313]

There is one related rule that I could find so far:

https://github.com/DavidAnson/markdownlint/blob/224987d727ebada36752074d2466a061c9e03ff0/doc/md051.md

Related issues DavidAnson/markdownlint#121, DavidAnson/markdownlint#1533

[vcsjones]

It should be pretty trivial to implement a markdownlint plugin (or find one that does it already) that checks for link 404s.