Deno code is really close to browser code. If you have ESM code that runs in the browser and doesn't need access to the DOM then it's a good bet it'll run on Deno, and vice versa. They use web standards for most things, and anything proprietary is put on the Deno global object. For standards that need adapting to work outside the browser, they're working with Cloudflare and others on WinterCG, which is defining a common baseline for these non-browser runtimes.