Reference
The authoritative support matrix. Three buckets throughout: ✓ supported, ~ supported with stated deviations, ✗ not supported (dropped gracefully unless noted). Anything not listed is unsupported.
Elements
| Element | Status | Notes |
|---|---|---|
div, span, p, h1–h6 | ✓ | block/inline per the UA stylesheet |
ul, ol, li | ~ | • markers via li::before; no ol counters yet |
blockquote, pre, code, hr | ✓ | |
strong/b, em/i, u, s/del, mark, kbd, abbr, samp, var | ✓ | SGR-attribute styling |
a | ✓ | underlined, focusable; Enter/click opens href |
button | ✓ | Enter/Space/click activate |
input (text) | ✓ | readline keymap, value, maxlength, readonly, password, bind:value |
input checkbox / radio | ✓ | [x] / (•), Space toggles, radio name groups, bind:checked |
select / option / optgroup | ~ | popup-less cycling control (no dropdown overlay), bind:value |
textarea | ✓ | multi-line editing, bind:value |
progress, meter | ✓ | eighth-block bars |
details / summary | ✓ | ▶/▼, open, toggle event |
dialog | ✓ | modal focus trap, Escape closes |
label | ✓ | wrapping and for= association |
region | ✓ | consumer-fed cell area + resize event |
ansi | ✓ | raw SGR passthrough, pre-like |
img | ✗ | planned (half-block cells, then kitty graphics) |
table, tr, td, th, thead/tbody/tfoot, caption, colgroup | ✓ | see Layout |
CSS: layout
| Feature | Status | Notes |
|---|---|---|
display: block / inline / inline-block | ✓ | real inline formatting context: runs wrap across styled elements |
display: flex | ✓ | direction, wrap, gap, grow/shrink/basis, justify (incl. space-around/evenly), align, align-self, order, reverse |
display: grid | ~ | cell/ch/%/fr, repeat(), minmax(), areas, span; row auto-flow only |
display: table + internals | ✓ | colspan/rowspan, caption, header/footer groups, border-collapse, border-spacing, table-layout: fixed, empty-cells, colgroup width hints |
display: contents / none | ✓ | |
width/height, min/max-*, %, calc() | ✓ | box-sizing both models |
| padding, margin | ✓ | margin auto centring; vertical margin collapse between adjacent block siblings (positive margins only) |
position: relative / absolute / fixed / sticky | ✓ | z-index paint order |
overflow: hidden / scroll / auto | ✓ | scrollbars, mouse wheel, scroll state |
| Units | ~ | cell and ch (1 cell) only; pixel-derived units drop to 0 |
CSS: selectors and cascade
| Feature | Status | Notes |
|---|---|---|
| tag / class / id / attribute selectors, combinators | ✓ | |
structural pseudo-classes (:first-child, :nth-child(), …) | ✓ | true runtime sibling context, including {for} lists |
:hover, :focus, :checked, :disabled, :enabled | ✓ | |
:not() / :is() | ~ | compound-only arguments |
::before / ::after | ~ | content strings, attr(), concatenation; subject-only |
| specificity + source order + inline-attribute precedence | ✓ | inline template attributes always win |
var() custom properties | ✓ | inheritance and fallbacks |
calc() / min() / max() / clamp() | ✓ | % resolves at layout |
@media | ✓ | viewport, prefers-color-scheme, prefers-reduced-motion |
@container | ✓ | size queries against laid-out ancestors |
@supports | ~ | property-name checks |
CSS: visual
| Feature | Status | Notes |
|---|---|---|
| colours | ✓ | named, hex, rgb(), light-dark(); Lab-space interpolation; quantized to detected depth (16/256/truecolor) |
opacity / alpha | ~ | composites over RGB backdrops; dim fallback otherwise |
| borders | ✓ | single/double/rounded/heavy/ascii box drawing; border-collapse junction merging; border-title (sumi extra) |
text-align, text-overflow (ellipsis, ellipsis-middle) | ✓ | |
white-space: normal / nowrap / pre, text-transform, word-break, visibility | ✓ | |
| transitions | ✓ | colours + lengths, easing incl. steps() |
@keyframes animations | ✓ | iteration/direction/fill-mode/play-state; per-node var() and light-dark() |
Events and input
| Feature | Status | Notes |
|---|---|---|
capture/bubble DOM dispatch, stopPropagation, preventDefault | ✓ | click, keydown, input, change, paste, toggle, focus, blur, resize |
| focus management | ✓ | Tab/Shift-Tab cycling, click-to-focus, dialog focus trap, disabled skipped |
| keyboard | ✓ | full modifier decoding, F1–F12, kitty protocol (disambiguate mode: Ctrl+Enter, clean Escape) with legacy fallback |
| mouse | ✓ | click, wheel scroll, hover, drag |
| global text selection | ✓ | drag / double-click word / triple-click line; inverse painting; OSC 52 + platform-tool clipboard on release |
| bracketed paste | ✓ | |
| Ctrl+Z suspend / resume | ✓ | clean terminal restore + repaint |
Runtime
| Feature | Status | Notes |
|---|---|---|
| fullscreen mode | ✓ | alternate screen, synchronized output (DEC 2026), diffed frames |
| inline mode | ✓ | renders at the shell cursor; final frame stays in scrollback; FrameLog streams and archives frames |
| io injection | ✓ | RunOptions.In/Out/Size; runs in the browser via runtime/webterm (wasm) |
| colour scheme | ✓ | OSC 11 detection, light-dark(), forced via option |
| reduced motion | ✓ | option or SUMI_REDUCED_MOTION |
| log capture | ✓ | RunOptions.OnLog routes stdlib log |
| tooling | ✓ | sumi init, sumi dev (hot reload, keep-last-good), sumi inspect |
Templates
| Feature | Status | Notes |
|---|---|---|
{expression} text, name={expr} / {name} attributes | ✓ | |
| mixed content (text interleaved with elements) | ✓ | whitespace-only gaps follow the JSX newline rule |
{if}/{else}/{/if}, {for} with key= diffing | ✓ | |
snippets and {render} | ✓ | |
components with props, callback props, bind:value, snippets | ✓ | |
{#await} / {#key} equivalents | ✗ |
Deviations from web behaviour are listed in each chapter. The terminal capability matrix (which terminals support which escape sequences) is in terminals.