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Strict TypeScript and Flow types for style based on MDN data


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TypeScript and Flow definitions for CSS, generated by data from MDN. It provides autocompletion and type checking for CSS properties and values.


import type * as CSS from 'csstype'; const style: CSS.Properties = { colour: 'white', // Type error on property textAlign: 'middle', // Type error on value };


// @flow strict import * as CSS from 'csstype'; const style: CSS.Properties<> = { colour: 'white', // Type error on property textAlign: 'middle', // Type error on value };

Further examples below will be in TypeScript!

Getting started

$ npm install csstype

Table of content

Style types

Properties are categorized in different uses and in several technical variations to provide typings that suits as many as possible.



  • All - Includes Standard, Vendor, Obsolete and Svg
  • Standard - Current properties and extends subcategories StandardLonghand and StandardShorthand (e.g. StandardShorthandProperties)
  • Vendor - Vendor prefixed properties and extends subcategories VendorLonghand and VendorShorthand (e.g. VendorShorthandProperties)
  • Obsolete - Removed or deprecated properties
  • Svg - SVG-specific properties


  • Default - JavaScript (camel) cased property names
  • Hyphen - CSS (kebab) cased property names
  • Fallback - Also accepts array of values e.g. string | string[]

At-rule types

At-rule interfaces with descriptors.

TypeScript: These will be found in the AtRule namespace, e.g. AtRule.Viewport.
Flow: These will be prefixed with AtRule$, e.g. AtRule$Viewport.


Pseudo types

String literals of pseudo classes and pseudo elements

  • Pseudos


    • AdvancedPseudos

      Function-like pseudos e.g. :not(:first-child). The string literal contains the value excluding the parenthesis: :not. These are separated because they require an argument that results in infinite number of variations.

    • SimplePseudos

      Plain pseudos e.g. :hover that can only be one variation.


All interfaces has two optional generic argument to define length and time: CSS.Properties<TLength = string | 0, TTime = string>

  • Length is the first generic parameter and defaults to string | 0 because 0 is the only length where the unit identifier is optional. You can specify this, e.g. string | number, for platforms and libraries that accepts any numeric value as length with a specific unit. const style: CSS.Properties<string | number> = { width: 100, };
  • Time is the second generic argument and defaults to string. You can specify this, e.g. string | number, for platforms and libraries that accepts any numeric value as length with a specific unit. const style: CSS.Properties<string | number, number> = { transitionDuration: 1000, };


import type * as CSS from 'csstype'; const style: CSS.Properties = { width: '10px', margin: '1em', };

In some cases, like for CSS-in-JS libraries, an array of values is a way to provide fallback values in CSS. Using CSS.PropertiesFallback instead of CSS.Properties will add the possibility to use any property value as an array of values.

import type * as CSS from 'csstype'; const style: CSS.PropertiesFallback = { display: ['-webkit-flex', 'flex'], color: 'white', };

There's even string literals for pseudo selectors and elements.

import type * as CSS from 'csstype'; const pseudos: { [P in CSS.SimplePseudos]?: CSS.Properties } = { ':hover': { display: 'flex', }, };

Hyphen cased (kebab cased) properties are provided in CSS.PropertiesHyphen and CSS.PropertiesHyphenFallback. It's not not added by default in CSS.Properties. To allow both of them, you can simply extend with CSS.PropertiesHyphen or/and CSS.PropertiesHyphenFallback.

import type * as CSS from 'csstype'; interface Style extends CSS.Properties, CSS.PropertiesHyphen {} const style: Style = { 'flex-grow': 1, 'flex-shrink': 0, 'font-weight': 'normal', backgroundColor: 'white', };

Adding type checked CSS properties to a HTMLElement.

import type * as CSS from 'csstype'; const style: CSS.Properties = { color: 'red', margin: '1em', }; let button = document.createElement('button'); Object.assign(, style);

What should I do when I get type errors?

The goal is to have as perfect types as possible and we're trying to do our best. But with CSS Custom Properties, the CSS specification changing frequently and vendors implementing their own specifications with new releases sometimes causes type errors even if it should work. Here's some steps you could take to get it fixed:

If you're using CSS Custom Properties you can step directly to step 3.

  1. First of all, make sure you're doing it right. A type error could also indicate that you're not :wink:

    • Some CSS specs that some vendors has implemented could have been officially rejected or haven't yet received any official acceptance and are therefor not included
    • If you're using TypeScript, type widening could be the reason you get Type 'string' is not assignable to... errors
  2. Have a look in issues to see if an issue already has been filed. If not, create a new one. To help us out, please refer to any information you have found.

  3. Fix the issue locally with TypeScript (Flow further down):

    • The recommended way is to use module augmentation. Here's a few examples:

      // My css.d.ts file import type * as CSS from 'csstype'; declare module 'csstype' { interface Properties { // Add a missing property WebkitRocketLauncher?: string; // Add a CSS Custom Property '--theme-color'?: 'black' | 'white'; // Allow namespaced CSS Custom Properties [index: `--theme-${string}`]: any; // Allow any CSS Custom Properties [index: `--${string}`]: any; // ...or allow any other property [index: string]: any; } }
    • The alternative way is to use type assertion. Here's a few examples:

      const style: CSS.Properties = { // Add a missing property ['WebkitRocketLauncher' as any]: 'launching', // Add a CSS Custom Property ['--theme-color' as any]: 'black', };

    Fix the issue locally with Flow:

    • Use type assertion. Here's a few examples:

      const style: $Exact<CSS.Properties<*>> = { // Add a missing property [('WebkitRocketLauncher': any)]: 'launching', // Add a CSS Custom Property [('--theme-color': any)]: 'black', };

Version 3.0

  • All property types are exposed with namespace
    TypeScript: Property.AlignContent (was AlignContentProperty before)
    Flow: Property$AlignContent
  • All at-rules are exposed with namespace
    TypeScript: AtRule.FontFace (was FontFace before)
    Flow: AtRule$FontFace
  • Data types are NOT exposed
    E.g. Color and Box. Because the generation of data types may suddenly be removed or renamed.
  • TypeScript hack for autocompletion
    Uses (string & {}) for literal string unions and (number & {}) for literal number unions (related issue). Utilize PropertyValue<T> to unpack types from e.g. (string & {}) to string.
  • New generic for time
    Read more on the "Generics" section.
  • Flow types improvements
    Flow Strict enabled and exact types are used.


Never modify index.d.ts and index.js.flow directly. They are generated automatically and committed so that we can easily follow any change it results in. Therefor it's important that you run $ git config merge.ours.driver true after you've forked and cloned. That setting prevents merge conflicts when doing rebase.


  • npm run build Generates typings and type checks them
  • npm run watch Runs build on each save
  • npm run test Runs the tests
  • npm run lazy Type checks, lints and formats everything



Last updated on 01 Apr 2023

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