Per discussion in primer/react#2339, the fact that this element automatically registers itself at the top level makes it incompatible with server-side rendering, where JavaScript might be run outside of a browser context.

From an SSR perspective, adding a simple typeof window !== 'undefined' guard should be sufficient. However, that won't solve the fact that the markdown-toolbar-element currently executes with side-effects, and thus cannot be tree-shaken. This means bundles in dotcom that do not need toolbar element will carry it as extra weight that is not needed. We would need to add a register() function to allow tree shaking, which would be a breaking change on the web component package.