Implicit default support values

[caugner]

What would you like to see added to BCD?

I'm proposing to introduce implicit default support values for certain browsers:

1. false (no support) for IE
2. "mirror" (same as upstream) for the following browsers:
   • chrome_android
   • firefox_android
   • oculus
   • opera
   • opera_android
   • safari_ios
   • samsunginternet_android
   • webview_android
   • webview_ios

Note

1. This is not a breaking change, because it only impacts the data stored in this repo, not the built and published BCD data.
2. This list intentionally excludes edge, which has significantly more non-mirrored statements (see below).
3. If we introduce statement-level annotations, we might still be able to set mirroring explicitly with a provenance:collector tag.

How impactful do you think this enhancement will be?

Overall, I believe that this change improves the authoring and reviewing experience.

As for IE, new web features are not supported for obvious reasons, so this entry just adds noise.

As for the mentioned downstream browsers:

• Implicit mirroring allows us to focus on the actual notable changes for these downstream browsers, so it reduces the chance of missing unintended "false" values during review, because these would then stand out, triggering the reviewer to look for corresponding evidence in the PR description.
• Currently, we seem to be adding "mirror" mostly without explicitly verifying this, because it is usually fair to assume that a feature supported in Chrome is also available in Chrome Android, and/or it isn't always straight-forward to verify.
• Recently, we've had some cases where false seems to have been set accidentally for some downstream browsers (ex 1, ex 2, ex 3, ex 4).

Do you have anything more you want to share?

According to rg ': "mirror"' | wc -l, we have 132,276 instances of mirror in BCD.

As for mirroring, here are mirroring stats as of 2026-01-30 (total 16,831 features):

Browser	Count	Share
chrome_android	13564	81%
firefox_android	14572	87%
oculus	14938	89%
opera	12646	75%
opera_android	11285	67%
safari_ios	14246	85%
samsunginternet_android	14464	86%
webview_android	12130	72%
webview_ios	14734	88%
edge	9713	58%

(Source: for x in chrome_android edge firefox_android oculus opera_android safari_ios samsunginternet_android webview_android webview_ios; do printf "| $x | "; rg '"'$x'": "mirror"' | wc -l; done)

As for IE, false is currently set for 10,293 features (68%).

(Source: npm run stats --all --count on 4daafe2)