Logging
There is no stdout in the browser so the simplest way is to use the log crate with wasm-logger and console_error_panic_hook to see nicely formatted errors:
#[wasm_bindgen(start)]
pub fn main() {
wasm_logger::init(wasm_logger::Config::default());
console_error_panic_hook::set_once();
// etc init and mounting of elements
log::info!("it works!");
}