Outlook 365 vs Outlook Classic: CSS Support Differences
Outlook 365 renders with Outlook Web; Outlook Classic renders with Microsoft Word. The gap runs almost entirely one way: 77 features work in Outlook 365 and break in the other, against 2 in the reverse direction. Build for the stricter one and the permissive one follows.
Support data derived from caniemail.com (MIT) · last verified Apr 27, 2026 · engine data v0.10.4
41
work in both
the safe intersection
77
only in Outlook 365
breaks in Outlook Classic
2
only in Outlook Classic
breaks in Outlook 365
49
partial on one side
renders, with caveats
Works in Outlook 365, breaks in Outlook Classic
Each of these needs a fallback if the send reaches both.
- <abbr>
- <acronym>
- <meter>
- <rp>
- <rt>
- <ruby>
- <select>
- <textarea>
- <wbr>
- @media
- align-items
- background-blend-mode
- background-clip
- background-image
- background-origin
- background-position
- background-repeat
- background-size
- border-radius
- border-spacing
- bottom
- box-shadow
- box-sizing
- caption-side
- clear
- column-count
- columns
- empty-cells
- fit-content
- flex-direction
- flex-wrap
- float
- font-size-adjust
- justify-content
- left
- list-style-position
- max-height
- min-height
- min-width
- mix-blend-mode
- object-fit
- object-position
- opacity
- orphans
- outline
- rgba
- right
- system-ui
- tab-size
- table-layout
- text-decoration-color
- text-decoration-line
- text-decoration-style
- text-emphasis
- text-orientation
- text-overflow
- text-underline-position
- top
- unit-ch
- unit-initial
- unit-rem
- unit-vh
- unit-vmax
- unit-vmin
- unit-vw
- visibility
- white-space
- widows
- word-spacing
- writing-mode
- z-index
- :adjacent-sibling
- :child-combinator
- :general-sibling
- :universal-selector
- display:flex
- display:grid
Works in Outlook Classic, breaks in Outlook 365
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>
- <form>
- <marquee>
- <style>
- <ul>
- !important
- background
- border
- display
- font-size
- font-weight
- gap
- letter-spacing
- line-height
- list-style
- margin
- max-width
- overflow
- padding
- position
- pseudo-class-active
- pseudo-class-default
- rgb
- text-align
- text-decoration
- text-justify
- text-transform
- word-break
- ::after
- ::before
- ::first-letter
- ::first-line
- ::placeholder
- :attribute-selector
- :checked
- :first-child
- :first-of-type
- :focus
- :hover
- :last-child
- :last-of-type
- :link
- :only-child
- :only-of-type
- :target
- :visited
- display:none
A further 78 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 247.
8 of 255 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 Properties84
- Pseudo-Classes & Selectors19
- HTML Elements17
- Pseudo-Elements5
- CSS At-Rules2
- CSS Functions1
24 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 Outlook 365.
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Questions
- How different are Outlook 365 and Outlook Classic?
- They disagree on 128 of the 247 features verified in both clients, or 52%. 41 features render in both and are the safe intersection to build on.
- What works in Outlook 365 but not Outlook Classic?
- 77 features, including <abbr>, <acronym>, <meter>, <rp>, <rt> and 72 more. Each needs a fallback if the send reaches both clients.
- Which engines are these two clients using?
- Outlook 365 renders with Outlook Web; 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 0.10.4 and last verified Apr 27, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.
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