This is a breakdown of "List of Suggested Extensions to CSS", produced by the CSSWG back in 1998. It goes through each feature and either lists where the feature has been defined in the intervening 16 years, or notes that it has yet to be addressed.
Out of the 67 suggested extensions, 30 have been added, 10 have been partially added or been added but with uncertain implementor interest, and 27 have not been added.
- ✓ Columns: http://dev.w3.org/csswg/css-multicol/
- ✓ Swash letters and other glyph substitutions: http://dev.w3.org/csswg/css-fonts/#font-rend-props
- ~ Running headers and footers: http://dev.w3.org/csswg/css-gcpm/#running-elements, but unproven so far
- ✗ Cross-references: not yet, but being investigated as part of footnotes
- ✓ Vertical text: http://dev.w3.org/csswg/css-writing-modes/
- ✓ Ruby: http://dev.w3.org/csswg/css-ruby/
- ✗ Diagonal text & text along a path: no plans yet, outside of SVG
- ~ Style properties for embedded 2D graphics: Kinda http://www.w3.org/TR/SVGParamPrimer/, but needs to be rebased over
var()syntax. - ✓ Hyphenation control: http://dev.w3.org/csswg/css-text/#hyphens-property, but could use more sophisticated controls
- ✓ Image filters: http://www.w3.org/TR/filter-effects/
- ✗ Rendering objects for forms: not yet, but lots of interest
- ✓ A pseudo-element for the URL #fragment-id: http://dev.w3.org/csswg/selectors/#the-target-pseudo
- ~ Floating boxes to top & bottom of a page: https://figures.spec.whatwg.org/, but implementor interest is unproven
- ✗ Footnotes: not yet, but strong interest
- ✗ "Tooltips"/"balloonhelp": no
- ✗ Math: no
- ✗ Folding lists: no, though IE's treatment of Regions (flowing the contents of an iframe into a region) is surprisingly similar
- ✗ Page-transition effects: not yet, but middling interest
- ✓ Timed styles: http://dev.w3.org/csswg/css-animations/
- ~ Leaders: http://dev.w3.org/csswg/css-gcpm/#leaders, but implementor interest is unproven
- ✗ Smart tabs: no (but this topic is tightly linked to leaders)
- ✗ Spreadsheet functions: no
- ✓ Non-rectangular wrap-around: http://dev.w3.org/csswg/css-shapes/
- ✓ Gradient & stretched backgrounds: http://dev.w3.org/csswg/css-images-3/#gradients and http://dev.w3.org/csswg/css-backgrounds/#the-background-size
- ~ Textures/images instead of foreground colors: no, but we plan to port the 'fill' property (which does this) from SVG to CSS
- ✓ Transparency: http://dev.w3.org/csswg/css-color/#transparency
- ✓ Constant expressions: http://dev.w3.org/csswg/css-values/#calc-notation
- ✓ Symbolic constants: http://dev.w3.org/csswg/css-variables/
- ✗ Mixed mode rendering: no
- ✗ Grid-device properties: no, but http://dev.w3.org/csswg/mediaqueries/#grid for detecting grid devices and http://dev.w3.org/csswg/css-values/#ch for measuring in character widths
- ✓ Co-dependencies between rules: http://dev.w3.org/csswg/css-conditional/#at-supports
- ✗ High-level constraints: no
- ✓ Float: gutter-side/fore-edge-side: http://dev.w3.org/csswg/css-page-floats/#floating-inside-and-outside-pages, but no implementor interest yet. Names ended up being "inside" and "outside".
- ~ Icons & minimization: http://dev.w3.org/csswg/css-ui/#icon, but implementor interest is unproven
- ✓ Namespaces: http://dev.w3.org/csswg/css-namespaces/
- ✗ Braille: no (but suggestions for MQs and properties would be appreciated)
- ✗ Numbered floats: no, but this is a separable piece of the "footnote" stuff
- ✗ "Visual" top/bottom margins: no
- ✗ ToC's, tables of figures, etc.: no
- ✗ Indexes: no
- ✗ ::nth-line() pseudo-element: no, but will likely show up in the future
- ✗ 'First-word' pseudo-element: no ("word" is a fraught concept internationally)
- ✓ Corner pieces for borders: http://dev.w3.org/csswg/css-backgrounds/#corners
- ~ Local and external anchors: http://dev.w3.org/csswg/selectors/deferred-for-level-5 (it was in Selectors 4, but got punted due to difficulties in defining it)
- ✓ Access to attribute values: http://dev.w3.org/csswg/css-values/#funcdef-attr
- ✓ Linked flows: http://dev.w3.org/csswg/css-regions/
- ✓ Pseudo-classes for user states: http://dev.w3.org/csswg/selectors/#drag-pseudos for drag/drop, but nothing else has been defined yet
- ✓ List numbering generalized and internationalized: http://dev.w3.org/csswg/css-counter-styles/
- ~ "Subtractive" text-decoration: no, but planned for next level of http://dev.w3.org/csswg/css-text-decor/
- ✗ Style for HTML's MAP & AREA elements: no
- ✗ Transliteration: no, but ideas based on text-transform have been bandied about seriously
- ~ Regular expressions in selectors: no in general, but http://dev.w3.org/csswg/selectors/#attribute-selectors for simple forms of this for attribute values
- ✓ Last-of... selectors: http://dev.w3.org/csswg/selectors/#the-nth-last-child-pseudo and co
- ✗ Control over progressive rendering: no
- ✓ Inline-block: http://dev.w3.org/csswg/css2/visuren.html#display-prop
- ✓ Non-breaking inline elements: http://dev.w3.org/csswg/css-text/#white-space-property
- ✗ Suppress word spacing: no
- ✓ HSV or HSL color notation: http://dev.w3.org/csswg/css-color/#the-hsl-notation
- ✓ 140-odd color names: http://dev.w3.org/csswg/css-color/#named-colors
- ✗ Copyfitting/auto-sizing/auto-spacing: no, but being currently discussed
- ✓ @page inside @media: http://dev.w3.org/csswg/css-page/#at-page-rule
- ~ Color profiles: kinda http://dev.w3.org/csswg/css-color/#color-management, but this is still undergoing major work. The use-case presented (256-color displays) is basically obsolete at this point, but there are new use-cases (wide-gamut images) that are still relevant.
- ✓ Underline styles: http://dev.w3.org/csswg/css-text-decor/#text-decoration-style-property
- ✗ Actions/behaviors mixed in with styles: no, but https://developer.mozilla.org/en-US/docs/Web/API/document.querySelector brought us a lot closer
- ✗ Comment syntax "//": no
- ✓ Replaced elements without an intrinsic size: http://dev.w3.org/csswg/css-images-3/#concrete-size-resolution
- ✓ Fitting replaced elements into a given space: http://dev.w3.org/csswg/css-images-3/#the-object-fit