Add no-unknown-custom-media

[jeddy3]

What is the problem you're trying to solve?

I'd like to disallow unknown custom media queries. For example:

@custom-media --foo (max-width: 30em);
@media (--bar) {}

What solution would you like to see?

A new rule to catch this. Like #6361, it's currently available as a plugin but it (perhaps) meets our criteria to be a built-in rule. Unlike #6361, it's for a draft spec. However, we already have a custom media queries rule (custom-media-pattern) and both PostCSS Preset Env and Lightening CSS offer syntax lowering for it.

Interestingly, the plugin also enforces using custom media queries through its always and always-known primary options. Shall we take the same approach or create two rules?:

• one that checks for unknown custom media queries
• and another that ensures matching queries use the corresponding name

I'm leaning towards the latter.

• Name: no-unknown-custom-media
• Primary option: true
• Secondary options: importForm & resolver?
• Autofixable: No
• Message: - Unexpected unknown custom media
• Description: "Disallow unknown custom media queries."
• Extended description: "This rule considers custom media queries defined within the same source to be known. Use the importForm secondary option to specify more sources."
• Section: "Avoid errors" -> "General"

[ybiquitous]

@jeddy3 Thanks for the proposal. The new rule sounds good to me. 👍🏼

Shall we take the same approach or create two rules?:

I agree with creating two rules; one is no-unknown-custom-media (avoiding errors), while another would be something like custom-media-*** (enforcing conventions). Although, it seems unclear whether the latter rule would be needed now.

[jeddy3]

I agree with creating two rules; one is no-unknown-custom-media (avoiding errors), while another would be something like custom-media-*** (enforcing conventions). Although, it seems unclear whether the latter rule would be needed now.

SGTM.

• Name: no-unknown-custom-media
• Primary option: true
• Secondary options: importFrom: [] - array of glob paths to .css files
• Autofixable: No
• Message: - Unexpected unknown custom media
• Description: "Disallow unknown custom media queries."
• Extended description: "This rule considers custom media queries defined within the same source to be known. You can use the importFrom secondary option to specify more sources."
• Section: "Avoid errors" -> "Unknown"

Labelling as ready to implement.

If anyone fancies contributing this rule, there are steps on how to add a new rule in the Developer guide.

[romainmenke]

We will provide a parser function for custom media in the media query parser.

parseCustomMedia('--foo screen and (min-width: 300px)')

This returns false or a CustomMedia instance.

CustomMedia has methods/properties for :

• the extension name (--foo)
• custom media has true or false keywords
• media query list

This should make it possible to handle custom media in the same way as regular media query lists.

Rules that handle @media should be easily updated to also handle @custom-media without changing the AST visitors.

[github-actions]

This issue is older than one month. Please ask before opening a pull request, as it may no longer be relevant.

[ybiquitous]

@fpetrakov Thank you for starting this issue! @romainmenke's package seems to provide parseCustomMedia(), so I believe there are no blockers. 👍🏼

[ybiquitous]

FYI, we cannot provide the importFrom secondary option because of a resolver problem. See also:

• Add resolver to configuration #4088
• Add importFrom secondary option to no-unknown-custom-properties #6746

[silverwind]

@custom-media seems to not be part of any CSS standard, I wonder why stylelint chose to even support it.

[romainmenke]

@custom-media seems to not be part of any CSS standard

It is part of this standard : https://www.w3.org/TR/mediaqueries-5/#custom-mq
Browsers are however not very eager to implement this (yet).

[silverwind]

Yeah, I guess they are more likely to just make variables work inside media queries than to implement another at-rule, I suppose 😆

[romainmenke]

No, they are even less eager to do that :D

But I should take the time in the near future to check if it can get unstuck. It is also the 4th most visited page on the postcss-preset-env repo.