Potato

A web browser written from scratch in C++20 for Windows. No dependencies, simple Win32 GDI rendering, some CSS but no JavaScript.

Back in the 2000s, I needed to move from C++ to work on a Web project. I wanted to learn CSS better, so I decided the best thing to do, was write my own Web Browser in C++.

Here is what I came up with, probably the worst web browser in the world. I try to keep it working well enough to display Wikipedia.

What is in the box

About 24,000 lines of C++20 with no third-party HTML or CSS library, split into a reusable engine and the browser that drives it:

• src/libwebvis — the rendering engine, built as a standalone static library. An HTML tokenizer and parser that survives real-world markup (implicit tag closing, raw-text elements, entity decoding, charset sniffing); a CSS engine with selectors, specificity, combinators, the cascade, media queries, calc() and custom properties; and block, inline, float, flex and table layout.
• src/potato — the browser: window, toolbar, address bar, navigation history, GDI/WIC painting and asynchronous loading over WinHTTP.

libwebvis has no platform dependency at all — it does not even link libdesktop. Everything it needs from the machine arrives through two interfaces in src/libwebvis/webvis.h that the host implements: webvis::device_context, a stateless drawing surface where every call carries its own coordinates and colours, and webvis::host for text measurement, resource loading and environment queries.

Text measurement sits on the host rather than the drawing surface on purpose: layout therefore needs no surface at all, which is what makes the headless modes below possible, and what lets libwebvis render a web view inside other applications.

Building

Requires Windows x64 and Visual Studio with the Desktop C++ workload. The vendored dd runtime locates Visual Studio and uses the CMake and Ninja that ship with it.

.\dd.ps1 build        # both configurations
.\dd.ps1 test         # build and run the suite
.\dd.ps1 launch       # build Release, then start the browser

Output is Exe\potato-64.exe, or potato-64d.exe for Debug.

Diagnostics

Potato is a GUI application, but every diagnostic mode writes to the console, so all of them can be scripted.

Lay out a local page, with no window and no message loop. Nothing is downloaded, so the same input always produces the same output — this is the mode to use when judging a layout change:

.\dd.ps1 layout --file test-files/page.html --width 1902 --dump 4

It prints the document size and stage timings, a summary of the box tree, and a scan for geometry no correct layout should produce: boxes that overflow the viewport, start left of the origin, have a negative size, are text with no height, or are an <img> with no width.

Extract Wikipedia's stylesheet variables for comparison against what the cascade produces:

.\dd.ps1 analyze-wiki-css

Exe\potato-64.exe --eval:<url> runs a real URL through the full loading and rendering path with the window parked offscreen. Because resources arrive in a different order every run the numbers are not reproducible; save the page and use layout if you need to compare.

Documentation

AGENTS.md — conventions for contributors and coding agents.

License

See LICENSE. The browser links libdesktop (LGPL-2.1-or-later) statically, so a distributed binary must let its users relink it against a modified libdesktop (LGPL-2.1 section 6). libwebvis does not link it.

Before distributing the browser, include libdesktop's notices, licence and corresponding source, including changes, plus the application source and build instructions or relinkable object files. Record the exact revisions, compiler and configuration, and verify a build with a modified libdesktop. Permit modification for the recipient's own use and reverse engineering to debug those modifications. A link to the latest source alone is not a relinking package.