HTML Elements

<rt>

HTML element · 14 supported, 1 partial, 5 unsupported across 21 clients

The <rt> element marks the pronunciation annotation in ruby text. Email client support is uneven , some clients render <rt> above the base text correctly, while others render it inline as adjacent characters, which can corrupt the visual reading order.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Unsupported
Gmail Android	mobile	Gmail Mobile	Partial
Gmail iOS	mobile	Gmail Mobile	Unsupported
Outlook 365	webmail	Outlook Web	Supported
Outlook (New)	desktop	Outlook Web	Unsupported
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Supported
Outlook Android	mobile	Outlook Mobile	Supported
Outlook for Mac	desktop	WebKit	Supported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Supported
Yahoo Mail Android	mobile	Yahoo	Supported
Yahoo Mail iOS	mobile	Yahoo	Supported
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Unsupported
Proton Mail	webmail	Proton	Supported
AOL Mail	webmail	AOL	Supported
Fastmail	webmail	Fastmail	Supported
Superhuman	desktop	Blink	Unknown

Client-by-client behaviour for <rt>

Fully supports <rt> (14): Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, Thunderbird, Proton Mail, AOL Mail, Fastmail.

Partial support (1): Gmail Android. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (5): Gmail, Gmail iOS, Outlook (New), Outlook Classic, HEY Mail. Plan fallbacks for these clients before relying on <rt> in production sends.

Behaviour unverified in: Superhuman.

When to use <rt> in email

Furigana for Japanese newsletter content.
Pinyin annotations in language-learning email products.
Phonetic hints in cross-language brand campaigns.

Rendering behaviour and edge cases

Apple Mail and modern Gmail render ruby correctly with <rt> stacked above the base.
Outlook on Windows often inlines <rt> content, breaking the visual hierarchy.
Mobile webmail clients are inconsistent , test extensively before relying on ruby in production.

Recommended fallback strategy

For mission-critical ruby, render the annotation as inline-block text positioned above the base using explicit margin/font-size, rather than relying on the <ruby>/<rt> default rendering.