DOMWorld only needs to use Node's `fs` module if you're adding a
filepath as a script/style tag. We can detect this case and run the
`require` inline such that in a browser this code won't execute.
These files will be used by both the web and node versions of Puppeteer.
Another name for this might be "core" but I don't want to cause
confusion with the puppeteer-core package that we publish at the moment.