Allow overlapping BCD keys

[ddbeck]

Note: Don't merge this without confirming with me first. I want to show this PR to MDN in a meeting Thursday, April 23, 2025, before merging. I'll strike out this message when that's done.

This PR allows BCD keys to exist in multiple features and it demonstrates the capability with a single key, css.properties.text-wrap-style.pretty.

css.properties.text-wrap-style.pretty already exists in text-wrap-style. This PR adds the key to a second feature, text-wrap-pretty. This seems pretty logical to me: text-wrap-style shows support for the property in general, where text-wrap-pretty represents the specific value of pretty, which you might use in two properties.

Up to now, we've enforced (but never documented) a limitation that a key can only be in one feature at a time. Last I talked to MDN—the original objector to key duplication—were open to allowing keys to span features. It seems like it'd be nice to introduce this to a single, low-risk pair of features before extending the practice elsewhere.

• Fixes Allow overlapping BCD keys without creating ambiguity #1173
• Fixes Include css.properties.text-wrap-style.pretty in text-wrap-pretty feature? #2886

[foolip]

It would be good to use another feature as the first one, since I'd like to fix #2543 at some point and probably remove some of the features.

I think a very safe and old case to pioneer with is HTMLModElement as part of both <ins> and <del>.

[ddbeck]

Over in #1173 (comment), I recapped the discussion on this concept (overlapping keys) from the WebDX call this week.

Concretely, I need to incorporate a few updates here:

• Allowlisting overlapping keys, so we can't overlap keys unintentionally
• Documenting authoring guidelines, so we know when to overlap keys
• Documenting expectations for consumers, so they know how to interpret this data sensibly