I am not an accessibility expert. Someone like Jason Knight would be more qualified to speak on this than I. As far as I can tell there are strategies and tools built into the browser to help specify the intent of a custom web component. From there I suspect that the regular accessibility concerns and strategies apply. Use semantic markup, try to keep presentation out of it, label stuff, use smart color contrasts, etc… But I’m no expert, so I’ll defer to my betters on this.