Setting a position-area example: issue with "center span-all"

[internet-account2]

MDN URL

https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Anchor_positioning/Using

What specific section or headline is this issue about?

#setting_a_position-area first example with the select element

What information was incorrect, unhelpful, or incomplete?

"center span-all" has no effect. Instead of being somehow positioned relative to the anchor, the infobox is placed into the center of the example page.

What did you expect to see?

I think the expected effect would have been: placed into center row, spanning all columns?
Or could it be the case that this setting is considered illegal because it could as well mean centered into center column, spanning all rows?

Do you have any supporting links, references, or citations?

My browser is Firefox 151.0.4 (64 bit) (archlinux: 151.0.4-1)

According to https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/position-area_value#browser_compatibility Firefox should be fully compatible since 148.

Do you have anything more you want to share?

Now I have no idea if this is simply a browser compatibility issue or some completely different issue.

So I think either the compatibility info is wrong or the "center span-all" setting is wrong/illegal (could there be any other option?) but I don't know which case it is.

MDN metadata

Page report details

• Folder: en-us/web/css/guides/anchor_positioning/using
• MDN URL: https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Anchor_positioning/Using
• GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/css/guides/anchor_positioning/using/index.md
• Last commit: 06e6e54
• Document last modified: 2026-01-25T19:35:57.000Z

[internet-account2]

Somewhat related to this:
setting position-area to just span-all (removing the first center) seems to place the infobox correctly (at least the way I thought center span-all was supposed to work): center row, spanning all columns.

Also somewhat related:
According to https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/position-area

position-area: center; /* equiv: center center */
position-area: span-all; /* equiv: center center */

span-all would be equivalent to center or center center. But the latter two show the infobox in the example with a smaller width (constrained to just the center tile instead of spanning the whole column?) so it doesn't seem equivalent at all?

(again observed in Firefox 151.0.4 (64 bit) (archlinux: 151.0.4-1), which should be fully compatible according to the compatibility table)