Hey great question! WebContext owns an output buffer (i.e., RenderBuffer) that components write HTML into directly, avoiding intermediate allocations and copying. It needs to be mutable for this reason. This is one of the key ways we're able to make Proa the fastest server-side rendering engine on the market.

By default, WebContext composes RenderBuffer (but it can be overriden via generics) which uses fixed-size 4 KiB pages. When a page runs out of room, it adds another page without moving the HTML already written. This avoids copying the accumulated output during growth! Its designed to be a performance improvement over Vec<u8>

For context, the problem with Vec<u8> is that every time a vector reaches maximum capacity, it has to reallocate a new area of memory (typically 2x larger than the previous allocation) and then copy everything into that new buffer. This adds a ton of overhead, especially if you're writing large pages.

Checkout https://github.com/Proa-Labs/proa/blob/384136f87fc9f91747477... if you're curious about RenderBuffer's design