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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: Restructuring this page for 2020.

Previously: Updating 2018 completions and reprioritizing for 2019

To Be Updated

The following sections of this page are out of date as of 2020-01-01 and kept only for historical purposes as part of restructuring this page for 2020.

If you have questions about any specific CSS feature, please click through to its bug/metabug and check its actual status there.


Tantek (tantek.com) 23:22, 17 January 2020 (UTC)

To Be Reprioritized

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


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

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)

Related HTML5

HTML5 layout and presentation related work:

Cascading and Inheritance

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)

new pseudo-classes

new properties

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)

Filter Effects Level 2

Inline Layout


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

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


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

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.


  • font inflation removed
  • CSS Transform properties (shorthands)


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


See Also