Define "from scratch" in "building a web browser from scratch". This thing has over 100 crates as dependencies... To implement css layouting, it uses Taffy, a crate used by existing browser implementations...
If one vulnerability exists in those crates well, thats that.
I can create a web browser in under a minute in Copilot if I ask it to build a WinForms project that embeds the WebView2 "Edge" component and just adds an address bar and a back button.