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This is the Mozilla wiki home page for Cascading Style Sheets (CSS), one of several areas of Open Web Standards.


Platform priorities

The following CSS specs/proposal are a priority for the Platform.

Actively Implementing


Mozilla has a pretty solid complete Flexbox (W3C CR, just waiting on feature-interaction tests to go PR) implementation. We are working on some fixes to make it even better, e.g.


Grid Level 2


Box Alignment


(Jen has a demo of bugs / test page at http://labs.jensimmons.com/examples/multicolumn-3-bug-demo.html)


3D Transforms

Inline Layout



CSS Intrinsic & Extrinsic Sizing Module Level 3 (WD)

Text Decoration

CSS Text Decoration Module Level 3 (2013 CR) (metabug), e.g.:

Writing Modes

CSS Writing Modes Level 3 (2015 CR) (metabug)

Under the hood and related


Font Rendering Controls

CSS Font Rendering Controls Module Level 1 (ED)

Font Loading

CSS Font Loading Module Level 3 https://developer.mozilla.org/en-US/docs/Web/API/CSS_Font_Loading_API shipped in 41

  • CSS Font Loading Improvements, Multiple bugs


  • font inflation removed
  • CSS Transform properties (shorthands)

Related HTML5

HTML5 layout and presentation related work:

Recent CRs

Cascading and Inheritance

Upcoming CRs

These CRs are expected in 2017:

For all potential CRs:

  • Which browsers have openly announced implementations or intent to implement? (links?)
  • Box Alignment (see above)

Text Level 3

Selectors 4

Selectors 4 (need list of changes since 3), metabug

  • No explicit dependency on Stylo, just lower priority
  • Selectors4 remaining features
    • CSS ::selection fixes + unprefixing
    • ... remaining new features? (JS)

Shipping Soon

As the above priorities get implemented, moving down here to note we're waiting for them to ship.

Post Stylo Integration

CSS Color Level 4

CSS Color Module Level 4 (ED), metabug

  • Color improvements (wide gamut, color correction, note CSS color correction preffed off)

Media Queries 4

Values and Units 3

CSS Values and Units Module Level 3 (CR)

  • [ ] attr function (at risk) — very useful in combination with CSS Shapes, in the context of a CMS

Image Values and Replaced Content

CSS Image Values and Replaced Content Module Level 3 (CR)

  • image() function
  • [ ] xywh fragment syntax

Style sheet APIs for Add-ons

  • style sheet APIs in add-ons SDK / loading async APIs (to be handled by stylo)

Filter Effects Level 2

Motion Path Module

https://www.w3.org/TR/motion-1/ (currently Working Draft)

Device Adaptation

CSS Device Adaptation Module Level 1 (WD)


  • CSS Houdini - see inside for Houdini implementation thoughts/plan


Please add subpages for each (unprefixed) CSS property in alphabetical order.

in development

CSS properties which have not yet made it to a Candidate Recommendation (or later), or are only implemented in the wild in prefixed form.

new pseudo-classes

new properties

see also