Yahoo Mail vs Outlook Classic: CSS Support Differences
Yahoo Mail renders with Yahoo; Outlook Classic renders with Microsoft Word. The gap runs almost entirely one way: 67 features work in Yahoo Mail and break in the other, against 3 in the reverse direction. Build for the stricter one and the permissive one follows.
Support data derived from caniemail.com (MIT) - last synced Sep 2, 2026 - engine data v0.12.2
49
work in both
the safe intersection
67
only in Yahoo Mail
breaks in Outlook Classic
3
only in Outlook Classic
breaks in Yahoo Mail
42
partial on one side
renders, with caveats
Works in Yahoo Mail, breaks in Outlook Classic
Each of these needs a fallback if the send reaches both.
- <button type="submit">
- <form>
- <input type="checkbox">
- <input type="hidden">
- <input type="radio">
- <input type="reset">
- <input type="submit">
- <input type="text">
- <rp>
- <rt>
- <ruby>
- <select>
- <textarea>
- <wbr>
- [background]
- @keyframes
- @supports
- amp4email
- background-position
- background-repeat
- background-size
- bimi
- border-spacing
- caption-side
- empty-cells
- image-maps
- list-style-position
- max-height
- min-height
- min-width
- opacity
- outline
- pseudo-class-active
- pseudo-class-default
- system-ui
- table-layout
- text-decoration-color
- text-decoration-line
- text-decoration-style
- text-shadow
- unit-vh
- unit-vw
- visibility
- white-space
- z-index
- ::first-letter
- ::first-line
- ::marker
- ::placeholder
- :adjacent-sibling
- :chaining
- :checked
- :child-combinator
- :first-child
- :first-of-type
- :focus
- :general-sibling
- :hover
- :last-child
- :last-of-type
- :only-child
- :only-of-type
- :target
- :universal-selector
- :visited
- display:flex
- display:grid
Works in Outlook Classic, breaks in Yahoo Mail
Partial on one side
These render in both, but one client applies caveats the other does not. They fail quietly, which makes them the expensive ones.
- <base>
- <body>
- <dfn>
- <marquee>
- <style>
- <ul>
- [dir]
- [lang]
- [role]
- [target]
- @media
- !important
- anchor-links
- background
- background-image
- border
- border-radius
- clear
- display
- float
- font-size
- font-size-adjust
- font-weight
- letter-spacing
- line-height
- list-style
- margin
- max-width
- overflow
- padding
- position
- rgb
- rgba
- text-align
- text-decoration
- text-indent
- text-justify
- text-transform
- transition
- word-spacing
- :attribute-selector
- display:none
A further 129 features render in neither client. Not a difference between them, but the other half of the answer to what you can actually use, so the four counts above deliberately do not sum to 290.
8 of 298 features have no confident data point for at least one of these clients, so they are excluded from every count above rather than assumed either way.
Where the divergence sits
Two pairs with the same headline number can be different problems. For these two it concentrates in CSS Properties.
- CSS Properties 56
- HTML Elements 21
- Pseudo-Classes & Selectors 19
- HTML Attributes 5
- CSS At-Rules 4
- Pseudo-Elements 4
21 of these already have a fix
Each links to a copy-paste snippet that keeps the feature working in Outlook Classic without dropping it in Yahoo Mail.
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Questions
- How different are Yahoo Mail and Outlook Classic?
- They disagree on 112 of the 290 features verified in both clients, or 39%. 49 features render in both and are the safe intersection to build on.
- What works in Yahoo Mail but not Outlook Classic?
- 67 features, including <button type="submit">, <form>, <input type="checkbox">, <input type="hidden">, <input type="radio"> and 62 more. Each needs a fallback if the send reaches both clients.
- Which engines are these two clients using?
- Yahoo Mail renders with Yahoo; Outlook Classic renders with Microsoft Word. Where two clients share an engine the divergence is usually configuration, and where they do not it is the engine itself.
- Is this measured in the real clients?
- No. These are documented support levels derived from caniemail.com (MIT-licensed), carried in @emailens/engine data version 0.12.2 and last synced Sep 2, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Jul 13, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.
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