I know that they’ve optimised this as much as they can, but it still seems to catch pretty much everybody by surprise that every single instance of a web component with a shadow DOM needs the site’s CSS reset added to it individually (or just skip it and keep forgetting that things like box-sizing will be inconsistent with your non-shadow-DOM styles). There’s no way to say “here’s my default styles” that will work consistently across the whole page once you start using web components. And then you have the !important fights between the web component and its contents as well.
Even though it’s been standard practice amongst web developers for 15+ years, the people working on the web platform seem to mostly act like CSS resets aren’t a thing.