Gecko is the only implementation supports non-standard caption-side value left, right, top-outside, and bottom-outside that's only in CSS 2 spec, but not in the final CSS 2.1.

We probably should remove the support for these non-stand value. That can simplify nsTableFrameWrapper's reflow code, and any table bugs about sizing with regarding table caption should be easier to fix.

For the top-outside and bottom-outside value, there is a note in "17.4.1. Caption position and alignment". https://drafts.csswg.org/css2/#caption-position

Note: CSS2 (1998) described a different width and horizontal alignment behavior. That behavior will be introduced in CSS3 using the values top-outside and bottom-outside on this property,

but I cannot find where they are defined.

Seems fine, do you plan to take this? Otherwise this could be a mentored bug... it's a matter of putting these values behind a pref for now, right? Or is it more urgent to get rid of the code?

(In reply to Emilio Cobos Álvarez (:emilio) from comment #2)

Seems fine, do you plan to take this? Otherwise this could be a mentored bug... it's a matter of putting these values behind a pref for now, right? Or is it more urgent to get rid of the code?

Right, it's not urgent to remove the code in layout. It's sufficient for this bug to:

1. Put those non-standard values behind a preference.
2. Annotate any failing tests with the preference on.
3. Send an intent to unship to dev-platform.

After this reaches the release channel, we can clean this up in another bug.

I don't plan to take this myself because I don't know how to modify inherited_table.mako.rs to hide part of the values behind a preference without further invetigation. It would be great if you are willing to mentor this bug assume you've already known how to do it.

Emilio, how do you feel about the scope described in comment 3?

After bug 1690706, this is a matter of:

• Adding a pref to StaticPrefList.yaml (layout.css.caption-side-non-standard.enabled?)
• On the non-standard variants, add a #[parse(condition)] section like this, checking for the new prefs.

That's the first step, then we need to push to try and fix or enable the pref for potentially-broken tests. But the two above steps should be pretty easy and I can help with the rest if needed.

For possible mentees: Please use the "Request information from" checkbox so I don't miss questions, or ping me on chat.mozilla.org (I'm emilio there as well).

https://hg.mozilla.org/mozilla-central/rev/cdfd35a25475

I'm a a bit sad to see left/right removed, but sure, if Blink/WebKit have no intention of supporting them I guess it's the right call...

That said, it should be easy to find examples of captions on the inline-start side in books/magazines etc, often with sideways text. So perhaps the CSSWG and other vendors would be interested in adding that if we can find photos of a few examples? I suspect it's probably been more than a decade since this was last discussed in the CSSWG. Who knows, perhaps the mood has shifted since then? (especially since Chrome is currently working on a new table implementation, TablesNG)

Fwiw, it appears left/right were discussed somewhat recently (2016) and resolved to "not define them normatively":
https://github.com/w3c/csswg-drafts/issues/497

Emilio, could you help send an brief "intend to unship" to dev-platform, please? I feel this bug is worth mentioning.

Sent, feel free to reply with more context if you think it's useful, I believe you have more context than me wrt these :)

Thanks for sending the mail :)

I have updated the caption-side page on MDN, BCD, and added to the 87 release notes.