[css-page-3] Allow custom property declarations in `@page` and margin rules

[cdoublev]

Chrome/FF (at least) allows custom properties in @page and margin rules although this is not allowed by CSS Paged Media 3. CSS Anchor Position 1 does not allow them in @try.

But I cannot think of a valid reason to exclude them.

[xiaochengh]

Edit: This comment is only about @try. Made it explicit.

The current reason is that allowing custom properties in @try basically means allowing any property there, which may break anchor positioning entirely.

For example:

.target {
  position: var(--x);
  --x: absolute;
  position-fallback: --fallbacks;
}

@position-fallback --fallbacks {
  @try { --x: static; }
}

[faceless2]

Of the 8 print-focused CSS engines that support custom properties at https://printcss.live, 7 of them support them in @page. In all of those, the @page context inherits values from :root.

[cdoublev]

Thanks for both feedback.

Maybe I should have created two separate issues... I was pretty confident that it was an oversight for @try. Would it work if custom properties are accepted only if there is one or more declaration of the other accepted properties? I move this question in a separate issue.

Based on current interoperability, it seems to be required to define that they are valid in @page. I am not sure if these modifications would be enough:

  4.3. @page rule grammar

  [...]

  In addition, the following rules apply:
  
-    - The @page rule can only contain page properties and margin at-rules.
-    - The margin at-rules can only contain page-margin properties.
+    - The @page rule can only contain page and custom properties and margin at-rules.
+    - The margin at-rules can only contain page-margin and custom properties.

  [...]

  4.4. Cascading in the page context

  [...]
  
+ Custom properties inherits from '':root''.