Implementing the WhatWG DOM in C++

The Document Object Model is something web developers take for granted. You call document.getElementById() and expect an element back. But when you are building a browser rendering engine from scratch in C++, you quickly realize the immense complexity hidden behind the WhatWG DOM Living Standard.

My project, Webelo, is an HTML visualizer and DOM library written purely in C++. The goal isn't just to parse HTML, but to implement the exact data structures and algorithms dictated by the WhatWG specification.

"The spec is the truth. Everything else is an approximation."

The core of the DOM is the node tree. Implementing this in C++ requires careful memory management, as nodes must maintain parent, child, and sibling pointers without creating circular memory leaks. I implemented the full Document Object Model tree structure, relying heavily on a strict preorder depth-first search (DFS) traversal algorithm.

// Example of Preorder DFS in Webelo
void traverseDOM(Node* root) {
    if (!root) return;
    
    // Process current node
    process(root);
    
    // Traverse children
    for (Node* child : root->childNodes) {
        traverseDOM(child);
    }
}

The preorder DFS is critical because it mirrors how elements are sequentially evaluated and rendered on the screen. It is also the foundation for queries like querySelector.

Beyond structural traversal, I implemented the event dispatch mechanism. Following the spec, an event (like a click) doesn't just happen at a node. It goes through a Capture phase (down the tree), a Target phase (at the node), and a Bubble phase (up the tree). Implementing addEventListener and dispatchEvent in C++ meant building an event queue and propagation system that perfectly matched the WhatWG flow.

Webelo serves as the foundational architecture for a larger, fully-featured browser rendering engine. You can explore the source code directly on GitHub.