font-weight

Client-by-client behaviour for font-weight

Fully supports font-weight (12): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Superhuman.

Partial support (3): Outlook (New), Outlook Classic, Yahoo Mail. Expect rendering quirks unique to each engine — partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

When to use font-weight in email

• Distinguishing headings from body copy without changing font family.
• Emphasizing inline content (offer codes, names, dates) inside paragraph text.
• Pairing display weights with system fallback fonts in cross-platform templates.

Rendering behaviour and edge cases

• If you load a custom font with @font-face, weights other than what you've actually loaded will not render — they fall back to system fonts at the closest weight.
• Outlook on Windows can substitute Calibri Light for any 300-weight declaration, sometimes producing visibly different geometry.
• Some clients render bold differently when the parent <strong> already provides bolding, leading to faux-bold doubling.

Recommended fallback strategy

Stick to a small set of widely supported numeric weights (400, 600, 700) and ensure the corresponding cuts are loaded with @font-face when using a custom font. Avoid mixing <strong> with explicit font-weight: bold on the same element to prevent double-bolding.

Fixes & Workarounds