Happy-dom is a module that provides a DOM-like abstraction that's pretty popular for test frameworks and server-side rendering, because it's a lot faster and lighter than more full-fledged browser-like options. By default, happy-dom doesn't support Deno, which is the basis for Val Town's runtime, but there's a fork that conveniently swaps out the thing it doesn't support, which is the Node.js vm module.

Another way to work with the DOM is to use domino.