BS5 follow-up: color="default" attribute renders effectively-unstyled across components

[malberts]

Historical chain:

• Bootstrap 3 shipped a neutral-grey button colour called default (.btn-default, .badge-default, etc.). BootstrapComponents was originally built against BS3 and exposed default as a valid value for color on Modal, Popover, Button, and Badge.
• Bootstrap 4 dropped the default colour family. The BS4 migration guide recommends replacing btn-default with btn-secondary; the same recommendation applied to alerts and badges.
• Bootstrap 5 kept BS4's recommendation and went further on badges, dropping the whole badge-<color> family in favour of text-bg-<color> utilities.
• BC 4.x and 5.x effectively shipped color="default" as a no-op at the framework-rule level. The PHP emitters continued to accept the value and emit btn-default / badge-default, but the BS4 framework had no rule for those classes. BC carried two partial fixes on top:
  • modules/ext.bootstrapComponents.alert.fix.css — full .alert-default { background-color: #f5f5f5; border-color: #ddd; color: #333; } (introduced in BC 1.2, 2018).
  • modules/ext.bootstrapComponents.button.fix.css — link-colour rules for a.btn-default and a.btn-outline-default (anchor-only; overrides MediaWiki's anchor colour).
• Neither fix defined .btn-default for non-anchor buttons or .badge-default for any badge. Net BC 5.x rendering of color="default":
  • Modal / Popover trigger buttons (<button class="btn btn-default">): browser-default <button> styling + BS4's .btn base. Cross-browser-inconsistent.
  • Button component (<a class="btn btn-default">): dark-text padded link (the link-colour fix applying here), no background, no border.
  • Badge (<span class="badge badge-default">): transparent label with .badge padding only.
  • Alert (<div class="alert alert-default">): the BC-shipped grey-on-grey box (#f5f5f5 / #ddd / #333).
• BC 6.0 (Migrate to Bootstrap 5.3 #93) ships the same rendering for color="default" under BS5. The *-default class is a no-op in BS5 too. The two partial fixes (.alert-default and a.btn-default / a.btn-outline-default link colours) are preserved unchanged. Net behaviour is identical to BC 5.x.

Why #93 didn't translate default

We investigated mapping default → secondary (or light) at the PHP layer so existing wikitext using the legacy value would render with proper BS5 styling under 6.0. Decided against it because:

• The "back-compat being preserved" was an effectively-unstyled rendering for Modal / Popover / Button-as-button / Badge, and a BC-owned grey box for Alert. Mapping to secondary (or anything) would have been an improvement — a behaviour change layered on top of the framework lift, not a lift change itself.
• Mixing "lift to BS5 framework" with "improve previously-broken rendering" muddies the PR's character. Without the translation, Migrate to Bootstrap 5.3 #93 is purely a framework lift: same PHP-level semantics as BC 5.x, just compiled against BS5 instead of BS4.

Options if we decide to act later

1. Keep as-is. color="default" remains a documented attribute value that renders as effectively-unstyled (with the two pre-existing partial fixes carrying their existing behaviour). Zero work, no behaviour change.
2. Drop default from the attribute manager's allowed-value list. Documented behaviour change: color="default" becomes invalid and falls back to the per-component default (primary for Button/Badge, info for Popover, no colour class for Modal). The two pre-existing partial fixes (.alert-default styled rule, a.btn-default link colour rule) become unreachable through the BC PHP emitters but stay in the CSS — separate cleanup choice on whether to also delete those rules.
3. Map to a visually meaningful BS5 class in the PHP layer. Sub-choice on the target: secondary (BS4 upstream recommendation, mid-grey filled) vs light (closer to BS3's actual visual character, light filled) vs outline-secondary (subtle outlined). Apply consistently to Modal / Popover / Button / Badge / Alert. The pre-existing partial fix CSS becomes unreachable (same cleanup choice as option 2).

Originally raised on #93.