This is definitely the way to go! The browser is not only good at network requests and sandboxing but comes with the most powerful layouting system and rendering engine.
So yes, leveraging browsers just for the front-end of a local-first app is such a good direction to go. Generally apps tend to bundle their own browser runtime to leverage this component. Even the lighter weight electron alternatives tend to either implement a part of the browser runtime themselves or at least need separate webview instances. But I already have a browser installed and why can't my local apps integrate with my installed browser to expose their functionality? This project looks like a good step into that direction.
So yes, leveraging browsers just for the front-end of a local-first app is such a good direction to go. Generally apps tend to bundle their own browser runtime to leverage this component. Even the lighter weight electron alternatives tend to either implement a part of the browser runtime themselves or at least need separate webview instances. But I already have a browser installed and why can't my local apps integrate with my installed browser to expose their functionality? This project looks like a good step into that direction.