This gist documents the setup I used to run visual diffs on the MDC Web demo pages for testing https://github.com/material-components/material-components-web/issues/1285. Currently it does nothing beyond take screenshots of the pages as they appear initially (i.e. it's probably not very useful for e.g. drawer demos). It should be feasible to do more on a per-case basis if we want.
This process currently doesn't directly access files from a material-components-web checkout at all, so really this should be usable for testing other front-end projects as well (just change urls.txt).