Meziantou.Framework

Limitations

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The library implements most of the web platform, but it is not a complete browser. The main differences are listed below; the WPT coverage gives the reason why each web-platform-test that does not pass fails.

Display and input

  • No window. The pages are rendered into images: your application takes the screenshots and plays the user with BrowsingContext.Input (see User input). There is no pinch zoom, and the text cannot be selected with the pointer. The delegated ink trails (navigator.ink) are not drawn, and there is no scroll performance timing (PerformanceScrollTiming).

  • Layout. The layout engine is internal: it runs when a web API reads the geometry, and its results are read through the web APIs only (CSSOM View, the computed styles, the observers; see Layout and geometry). It does not implement:

    • the layout of the content of a page whose inline size differs from the previous page when the page starts inside a line box, a flex, grid or table container or a multi-column container: it keeps the inline size of the previous page;
    • the repetition of a hyphen at the start of the next line;
    • the spelling changes at a hyphenation break, except in Dutch, Hungarian and Pinyin;
    • the hyphenation dictionaries under copyleft licenses (cs, hu, hy, lv);
    • the scrolling of a ::scroll-marker-group that is a scroll container;
    • in the shaping of the complex scripts, the Myanmar Zawgyi encoding and the Arabic fallback shaping of the Windows-1256 fonts.

  • Painting. The core library paints the documents into a display list; a rendering backend produces the pixels. Without a backend, nothing is painted, the images are not decoded and canvas.getContext("2d") returns null. The Imaging backend does not paint:

    • the SVG color glyphs (the SVG table of a font): they are drawn as their outlines (the COLR glyphs, versions 0 and 1, and the bitmap glyphs of sbix and CBDT are drawn with their colors);
    • the wrapping of the SVG texts in the shapes of shape-inside;
    • the canvas layers (beginLayer()).

    Float16 canvas storage and transfers are supported, but gradients and intermediate effects use normalized 16-bit colors (see Canvas). As in Blink, the painted boxes and the baselines are snapped to the device pixels and the font metrics are rounded to whole pixels; the layout keeps its fractional geometry.

Images and media

  • Images are decoded by the rendering backend. Without a backend, only the natural dimensions of the images are read from their header: decode() resolves once the image is available, and createImageBitmap() rejects the images and the blobs. The Imaging backend decodes PNG (including animated PNG), JPEG (including CMYK), GIF, WebP, AVIF, JPEG XL, BMP and ICO images, with their EXIF orientation and resolution and their color profile. The pages are sRGB: the colors of an image outside of the gamut of sRGB are clipped, and only the Display P3 and half-float canvases keep them (the images whose color profile has lookup tables and the cICP descriptions other than Display P3 are clipped to sRGB there too).
  • Audio and video are not decoded. The media elements run on a media backend that your application provides (BrowsingContextOptions.MediaBackend); without it, they have no media. The Media Source Extensions parse the segments with the backend but do not decode the frames. The media controls and Encrypted Media are not implemented.

Network and security

  • Nothing is fetched without a document loader, and no script runs without a script engine: the scripts are not fetched and the event handler attributes have no effect. A document parsed with HtmlDocumentParser has no browsing context: no window, no computed style and no event loop, so the mutation observer callbacks are not invoked.
  • No downloads, and the third-party cookies are not blocked.
  • Reporting API. Content Security Policy reports its violations, but the Reporting API generates only the CSP, COEP and test reports: there are no deprecation, intervention, COOP or Document Policy reports.

Scripting

  • The streams cannot be transferred between workers or windows.
  • WebAssembly threads are not supported.
  • The scripts run in your process: read Running untrusted pages before you load pages you do not control.

Experimental APIs

The members that implement proposals that no specification defines yet (such as Element.StreamAppendHtml()) have the [Experimental] attribute: using them reports an error with a diagnostic per feature (WEBPLATEXP0002...), which you suppress to use them (<NoWarn>$(NoWarn);WEBPLATEXP0002</NoWarn>). They may change or be removed when the proposal changes.