Svelte author here. If you're using a bundler integration like rollup-plugin-svelte or svelte-loader (for webpack), those repeating lines of code are deduplicated across components. There's a bare minimum of repetition, and we're in the process of reducing it further. You'd be surprised at just how well it scales!

Browser inconsistencies are much less of an issue than they used to be. There are only a couple of places (e.g. listening for both 'input' and 'change' events on <input type='range'>, to satisfy IE and everyone else) where we need to accommodate differences.