[css-conditional-5] Consider deprecating and removing `CSSContainerRule.containerName` and `CSSContainerRule.containerQuery`

[cdoublev]

In #10845, the CSSWG resolved on adding CSSContainerRule.conditions to represent the list of @container conditions, and to make CSSContainerRule.containerName and CSSContainerRule.containerQuery, which were defined before changing the grammar to accept more than one name, return the empty string if the list has more than one condition.

This change was made with the following note:

Caution

We should try to remove containerName and containerQuery, since they don’t deal with multiple conditions correctly.

@emilio, who asked me to create this issue, added use counters to check if back-compat allows this.

In my opinion, deprecating them could also be an appropriate action to consider. Returning empty string when there is more than one name/query shows that they are half broken. This would also send a signal (eg. via MDN) to not use them, so that they can be removed.

[SebastianZ]

In my opinion, deprecating them could also be an appropriate action to consider.

If compatibility doesn't allow to remove them, that's probably the best option.

Returning empty string when there is more than one name/query shows that they are half broken.

If they need to be kept for web compatibility, we should check how they are used and depending on that decide whether they should return an empty string or maybe the first item in the list.

Sebastian