:universal-selector

Client-by-client behaviour for :universal-selector

Fully supports :universal-selector (10): Gmail, Outlook 365, Outlook iOS, Outlook Android, Apple Mail, Apple Mail iOS, Yahoo Mail, Samsung Mail, Thunderbird, HEY Mail.

Partial support (2): Gmail Android, Gmail iOS. Expect rendering quirks unique to each engine — partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (2): Outlook (New), Outlook Classic. Plan fallbacks for these clients before relying on :universal-selector in production sends.

Behaviour unverified in: Superhuman.

When to use :universal-selector in email

• Setting box-sizing: border-box across all elements in modern clients.
• Resetting margin and padding globally as the start of a custom design system.
• Applying a single dark-mode color override to every element via *.

Rendering behaviour and edge cases

• Specificity is the lowest possible — any other selector overrides *.
• Outlook on Windows ignores * because <style> is stripped.
• Universal selectors can have measurable rendering cost on slow clients with large emails.

Recommended fallback strategy

Avoid relying on * for production emails. If you need a global reset, apply explicit inline styles on each element instead. Reserve the universal selector for niche progressive-enhancement cases in modern clients only.

Fixes & Workarounds