Side-by-side coverage of the HTML5 Canvas WebIDL surface. Source of truth is COMPATIBILITY.md and the per-interface specs under specs/.

Legend

Mark	Meaning
✅	Fully implemented per spec
🟡	Partial / accepted-but-different / spec divergence (see notes)
❌	Not implemented yet (planned)
⛔	Out of scope (DOM / Blob / MediaStream / encoder dependencies)

Summary

Class	Browser	simdra	Headline gap
HTMLCanvasElement (Canvas)	✅	🟡	toBlob needs Node Blob shim; WebP encoder missing (no stb path); JPEG done via stb_image_write
CanvasRenderingContext2D	✅	✅	—
ImageData	✅	✅	Exposes Uint8Array instead of Uint8ClampedArray; colorSpace informational only
Image (Image.fromBytes)	✅ (HTMLImageElement)	🟡	Sync fromBytes factory rather than <img src=…> + onload; no async loader, no DOM lifecycle
Path2D	✅	🟡	Path2D(d) SVG ctor; arc/arcTo/ellipse/roundRect on Path2D
DOMMatrix	✅	🟡	setMatrixValue(transformList) + string-form ctor (CSS parser out of scope)
CanvasGradient	✅	🟡	createConicGradient
CanvasPattern	✅	✅	—
TextMetrics	✅	🟡	Only width populated; bbox / baseline fields not implemented
OffscreenCanvas	✅	❌	Whole interface unimplemented (low priority — Node has no worker transfer)

HTMLCanvasElement (Canvas)

Member	Browser	simdra	Notes
width	✅	✅	Read/write; assignment reallocates the bitmap (transparent black) and resets ctx state per spec
height	✅	✅	Same as width
getContext('2d')	✅	✅	Caches on first call; only '2d' supported
getContext(type, attrs)	✅	🟡	Attributes accepted and ignored
toDataURL()	✅	✅	Emits image/png
toDataURL(type)	✅	🟡	Recognizes image/png and image/jpeg; falls back to png for unrecognized types
toDataURL(type, quality)	✅	🟡	PNG + JPEG via stb_image_write; WebP still ⛔ (no stb path)
toBlob(...)	✅	⛔	Needs Node Blob shim; encode primitives are in place via Canvas.toBytes(type, quality)
transferControlToOffscreen()	✅	⛔	Browser worker transfer model
captureStream()	✅	⛔	MediaStream API
Event listeners (contextlost, …)	✅	⛔	DOM-only

simdra extensions (non-spec)

Member	Notes
Canvas.toBytes(type?, quality?)	Same dispatch as toDataURL but skips the base64 round-trip
Image.fromBytes(bytes)	Decoded image source for drawImage / createPattern. Backed by stb_image (PNG / JPEG / BMP / GIF first frame). Browser-shaped helper but not the spec's HTMLImageElement (no src / onload; bytes go in synchronously)
microsharp named export	Sharp-shaped fluent surface on the same Zig core. Accepts Uint8Array / ArrayBuffer / Blob / ReadableStream / Response
createCanvas(w, h, { fonts: [{ name, data, weight?, style? }] })	Optional 3rd arg — pin custom faces at construction time
registerFont(bytes, family, descriptor?)	Top-level equivalent of the constructor's fonts option. Mirrors node-canvas / @napi-rs/canvas

Fonts

	Browser	simdra
ctx.font shorthand	full CSS font shorthand	size + family + weight + style; font-variant, font-stretch, font-size-adjust accepted but no-op
Font registration	system fonts	embedded default (Manrope variable, ~162 KB, OFL 1.1) + registerFont for custom
Faux bold/italic	partial (Skia synthesises)	when no real face matches, faux-bold (1 px horizontal smear) + faux-italic (12° row shift)
font-variant (small-caps, etc.)	full	accepted, no effect
Variable axes (wght/wdth)	full	not yet wired (uses each axis's default instance)
Complex scripts (Arabic, Devanagari)	full	no shaping; latin / CJK punch-and-place only

Spec drift / known caveats

• ImageData.data is a Uint8Array view rather than Uint8ClampedArray. Mutation with out-of-range values will not be clamped on assignment — values are stored modulo 256. This will be fixed once node-zigar's typed-array marshalling supports clamped views.
• Color spaces (colorSpace: 'display-p3') are accepted on the ImageData constructor and getImageData(..., {colorSpace: ...}) but currently informational — all rasterization is sRGB.
• globalCompositeOperation supports the full Porter-Duff set; CSS-color-3 modes (multiply, screen, overlay, etc.) are implemented but slower than the Skia/Cairo equivalents pending SIMD-tuning.
• Text shaping is stb_truetype's punch-and-place — no kerning beyond the kern table, no GSUB ligatures, no BiDi, no complex script support. Latin and CJK are fine; everything else needs a real shaper (HarfBuzz integration is a roadmap item).

For the line-by-line view, see COMPATIBILITY.md in the repo.