document.registerElement is part of Custom Elements v0. That is being slowly deprecated and removed from the web platform in favor of the consensus standard Custom Elements v1 [1].

It was finally removed from recent versions of Chromium/Chrome, and thus also recent versions of Electron.

(It was removed in Chromium/Chrome 86 [2], which then carries over to Electron 11 [3])

Atom has an open PR exploring the potential move to Electron 11 (atom/atom#22687 (comment)), but this means a breaking change dropping document.registerElement support from Atom. I thought I would post an issue here to let you folks know about it, since I understand this is used by some other important packages in the Atom ecosystem.

Here is a comment with some resources about Custom Elements v1:
atom/atom#22011 (comment)

• fix: use customElements atom-community/terminal#70
• Migrate styles element to custom elements atom/atom#22749
• Use custom element on pane container element atom/atom#22750
• Use custom element on pane-element atom/atom#22751
• Use custom element on workspace-element atom/atom#22755
• Use custom elements on TestPanelContainerItemElement atom/atom#22769
• Use customElement for the specs atom/atom#22774
• Use custom elements on pane container element atom/atom#22780
• Use customElement on pane axis atom/atom#22785
• Use custom element on pane resize-handle element atom/atom#22787
• Use custom elements on text editor element spec atom/atom#22789
• Use custom element on text editor element atom/atom#22867

Some specs might need some modification to attach things to the DOM? For example:

• Attach elements to jasmine DOM atom/tree-view#1392