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CSS is short for Cascading Style Sheets (CSS), and typically includes the full set of Selectors, CSS properties, values, and @-rules supported by browsers.

CSS is one of several web standards.

For documentation on CSS, how to develop for it, use various properties, see:




The following CSS specs/proposals are a priority.

In Progress: Updating 2018 completions and reprioritizing for 2019

2018 Priorities

These are *in progress* 2018 Priorities for CSS features in Firefox.

Close to shipping

These features have implementations that are already well in progress and thus it is a priority to finish them and ship.

Grid Level 2

Grid Level 2, in particular subgrid (use-cases, a11y implications)

Values and Units 3

CSS Values and Units Module Level 3 (CR) metabug

  • [ ] calc() function in particular. metabug
  • re-prioritize Values and Units 3 once calc fixes have landed.
  • [ ] attr function (at risk) — very useful in combination with CSS Shapes, in the context of a CMS

Media Queries

2018 Medium Priorities

2018 medium priorities are "maybe" items that we're not sure we can get to in 2018, but would really like to (had a lot of interest during yallhands).


2018 Lower Priorities

Features or areas that are lower priority based on a number of factors.

Actively Implementing

Beyond the above priorities, we should continue to actively implement, fix bugs on the following:


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.


3D Transforms


Text Decoration

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

Writing Modes

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

Fonts Level 3

Wrap-up the few remaining bugs, or declare that we're done (e.g. font-variant as a descriptor?)

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.

Image Values and Replaced Content

[1] (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)

Device Adaptation

CSS Device Adaptation Module Level 1 (WD)


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

2019 Priorities

Resize Observer

  • Resize Observer (bugzilla, intent to implement coming)

Container Queries Prerequisites

  • This is a high priority to evaluate how workable it is to polyfill/prototype once we have Resize Observer (see above) implemented and working

There's a lot of anecdotal demand for Container Queries / Element Queries. See replies here:

We should at least figure out what features / bug fixes we would need to address to even consider something like Container Queries (which we should also give input on)

Scroll Snap update

Scroll Anchoring


CSS Intrinsic & Extrinsic Sizing Module Level 3 (TR WD) metabug

  • intrinsic size keywords: fix #567039 & #1055887, and unprefix (#1322780
    • 'min-content', 'max-content' in particular (needs bugzilla links)
    • re-evaluate priority at this point, e.g. 'fit-content' is lower priority
  • medium mobile impact from height related properties

Shadow Parts

CSS Shadow Parts (TR WD)

2019 Medium Priorities

These are *in progress* 2019 Priorities for CSS features in Firefox.

Filter Effects Level 2

Inline Layout


  • CSS Houdini - see inside for Houdini implementation thoughts/plan
  • working towards (longer-term)

2019 Lower Priorities

Box Alignment

  • CSS Box Alignment (WD), metabug
    • all properties for blocks (e.g. 1105571)
    • and some details for flexbox (see metabug 1105570 Depends on)
    • spec CR depends on resolving open css-align-3 issues
  • unlikely mobile impact (mobile layouts more likely to use flexbox with box alignment properties)


Conic Gradients

Shipped and evaluate follow up

Variable Fonts

Fonts Level 4 for Variable Fonts support in particular:

Reprioritize Fonts Level 4 accordingly once we have Variable Fonts.


2019 Needs Reprioritizing

These are all items we made progress in 2018 (in some cases shipping features or large portions there of) and need to re-evaluate what’s left (if anything) for 2019


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


Former 2018 Lower Priorities, need re-evaluation for 2019:

CSS Images 3

SVG properties in CSS

This bit from SVG2:

Motion Path

CSS Color Level 4

CSS Color Module Level 4 (ED), metabug

  • Color improvements (wide gamut, color correction, note CSS color correction preffed off)
  • low mobile impact (variable lighting anyway, users use of filters)

Implement correct color management of CSS colors (which is really earlier levels of CSS color).

Better Print Support

Start looking at what specs and features would significantly improve print support. Much of this is testing and bugfixing edgecases of existing features, some of it may require or benefit from new specs/features.

More specs / features in particular TBD.

See Also